Fix pydict serialization for optional fields

[nickderobertis]

What

Adds a block in Mesaage.to_pydict that explicitly checks for None values and assigns them to the dict if include_default_values=True.

Why

Fixes #475 where to_pydict fails on optional fields with the error:

  File "...", in <listcomp>
    json.dump([f.to_pydict(casing=Casing.SNAKE, include_default_values=True) for f in frames], file)
  File "...\Python\Python310\lib\site-packages\betterproto\__init__.py", line 1365, in to_pydict
    output[cased_name] = value.to_pydict(casing, include_default_values)
  File "...\Python\Python310\lib\site-packages\betterproto\__init__.py", line 1359, in to_pydict
    value._serialized_on_wire
AttributeError: 'NoneType' object has no attribute '_serialized_on_wire'

Testing

Added a test case that checks to_pydict and to_dict output when passing the message value and not, and when passing include_default_values and not.

[Gobot1234]

Thank you for the PR

[nickderobertis]

Thanks for reviewing and merging it quickly! When do you think we'll cut a new release with this change? Trying to see if I need to peg to git version or if it will be available in a v2 beta release soon.

[Gobot1234]

I can't make any guarantees as I can't make releases by myself so I'll have to recommend that you pin to a git rev, sorry.

[nickderobertis]

Ok that makes sense, thank you for the response.