Optional nullable model properties are deserialized from `UNSET` to `None`

[forest-benchling]

Describe the bug
If a model has an optional nullable model property, then in the from_dict, if the key is not present in the dictionary, it is deserialized as None instead of UNSET.

This is also true for enum properties, and probably other types as well, but we haven't done an exhaustive search.

To Reproduce
This can already be seen in end_to_end_tests/golden-record/my_test_api_client/models/a_model.py:345, where the deserialized value is initialized to None.

Expected behavior
UNSET keys should be deserialized as UNSET.

OpenAPI Spec File
Existing openapi.json

Desktop (please complete the following information):

• OS: [e.g. macOS 10.15.1]
• Python Version: [e.g. 3.8.0]
• openapi-python-client version [e.g. 0.1.0]

Additional context

[forest-benchling]

Sorry @dtkav for the barrage of feedback :) We are hoping to help out with some of it eventually, but don't have the bandwidth at the moment.

LMK if anything is unclear.

cc @dtkav @bowenwr @packyg

[dbanty]

@forest-benchling does this affect main, the released version, or both?

[forest-benchling]

@forest-benchling does this affect main, the released version, or both?

@dbanty Both. I think it has been around for a long time. It should be pretty easy to fix, though, it might just work to switch the order of checking optional/nullable in openapi_python_client/templates/property_templates/property_macros.py.jinja:6.