DyanmoDB: empty update expression in update_item raises the wrong exception

[felixscherz]

Hi, thanks for maintaining this package!

I was working with DynamoDB and found a bug that occurs when the update expression given to Table.update_item is an empty string "".

How to reproduce

import boto3
import moto

with moto.mock_aws():
    dynamodb = boto3.client("dynamodb", "us-east-1")

    dynamodb.create_table(
        TableName="foo",
        KeySchema=[
            {"AttributeName": "pk", "KeyType": "HASH"},
        ],
        AttributeDefinitions=[
            {"AttributeName": "pk", "AttributeType": "S"},
        ],
        BillingMode="PAY_PER_REQUEST"
    )

    dynamodb.put_item(TableName="foo", Item={"pk": {"S": "foo"}})
    dynamodb.update_item(
        TableName="foo",
        Key={"pk": {"S": "foo"}},
        UpdateExpression="",
    )

raises an AttributeError:

Traceback (most recent call last):
  File "/private/tmp/moto-dynamodb/t.py", line 19, in <module>
    dynamodb.update_item(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/client.py", line 565, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/client.py", line 999, in _make_api_call
    http, parsed_response = self._make_request(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/client.py", line 1023, in _make_request
    return self._endpoint.make_request(operation_model, request_dict)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/endpoint.py", line 119, in make_request
    return self._send_request(request_dict, operation_model)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/endpoint.py", line 200, in _send_request
    while self._needs_retry(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/endpoint.py", line 352, in _needs_retry
    responses = self._event_emitter.emit(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/hooks.py", line 412, in emit
    return self._emitter.emit(aliased_event_name, **kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/hooks.py", line 256, in emit
    return self._emit(event_name, kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/hooks.py", line 239, in _emit
    response = handler(**kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/retryhandler.py", line 207, in __call__
    if self._checker(**checker_kwargs):
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/retryhandler.py", line 284, in __call__
    should_retry = self._should_retry(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/retryhandler.py", line 307, in _should_retry
    return self._checker(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/retryhandler.py", line 363, in __call__
    checker_response = checker(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/retryhandler.py", line 247, in __call__
    return self._check_caught_exception(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/retryhandler.py", line 416, in _check_caught_exception
    raise caught_exception
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/endpoint.py", line 276, in _do_get_response
    responses = self._event_emitter.emit(event_name, request=request)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/hooks.py", line 412, in emit
    return self._emitter.emit(aliased_event_name, **kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/hooks.py", line 256, in emit
    return self._emit(event_name, kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/hooks.py", line 239, in _emit
    response = handler(**kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/core/botocore_stubber.py", line 37, in __call__
    response = self.process_request(request)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/core/botocore_stubber.py", line 84, in process_request
    status, headers, body = method_to_execute(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/core/responses.py", line 290, in dispatch
    return cls()._dispatch(*args, **kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/core/responses.py", line 501, in _dispatch
    return self.call_action()
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/utilities/aws_headers.py", line 44, in _wrapper
    response = f(*args, **kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/dynamodb/responses.py", line 196, in call_action
    return super().call_action()
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/core/responses.py", line 587, in call_action
    response = method()
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/dynamodb/responses.py", line 928, in update_item
    item = self.dynamodb_backend.update_item(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/dynamodb/models/__init__.py", line 560, in update_item
    item.update_with_attribute_updates(attribute_updates)  # type: ignore
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/moto/dynamodb/models/dynamo_type.py", line 368, in update_with_attribute_updates
    for attribute_name, update_action in attribute_updates.items():
AttributeError: 'NoneType' object has no attribute 'items'

Expected outcome

When running the request against AWS directly a ClientError is raised:

Traceback (most recent call last):
  File "/private/tmp/moto-dynamodb/t.py", line 35, in <module>
    dynamodb.update_item(
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/client.py", line 565, in _api_call
    return self._make_api_call(operation_name, kwargs)
  File "/private/tmp/moto-dynamodb/venv/lib/python3.10/site-packages/botocore/client.py", line 1017, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the UpdateItem operation: Invalid UpdateExpression: The expression can not be empty;

I had a look at the code already and I will submit a pull request with a possible fix shortly!