[V4] Update DocumentModel to handle expressions without variables

[dscpinheiro]

Fixes #3802

Motivation and Context

Fixes a regression introduced in V4 where an expression without variables would throw an AmazonDynamoDBException: ExpressionAttributeValues must not be empty. For example, this operation:

var ddb = new AmazonDynamoDBClient();
var table = new TableBuilder(ddb, "TestTable").AddHashKey("pk", DynamoDBEntryType.String).Build();
var doc = new Document { ["pk"] = Guid.NewGuid().ToString() };

await table.PutItemAsync(doc, new PutItemOperationConfig
{
    ConditionalExpression = new Expression { ExpressionStatement = "attribute_not_exists(pk)" },
});

Would result in the following request body:

{"ConditionExpression":"attribute_not_exists(pk)","ExpressionAttributeValues":{},"Item":{"pk":{"S":"386ba1d5-5b18-47e5-9c22-3b73e0f1ddd1"}},"TableName":"TestTable"}

This PR updates the document model to match the request sent by V3:

{"ConditionExpression":"attribute_not_exists(pk)","Item":{"pk":{"S":"75fbb295-a11c-4447-ac1a-bacc2baacd0c"}},"TableName":"TestTable"}

Testing

• Dry-run: DRY_RUN-c628983c-fa45-42c4-a4d0-81921c146e4f
• Also confirmed this works when AWSConfigs.InitializeCollections is set to true

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the code style of this project
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license