fix: Don't crash when a null is in an enum (fixes #500). Thanks @juspence!

[dbanty]

No description provided.

[codecov]

Codecov Report

Merging #501 (8779620) into main (5861c7c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #501   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           48        48           
  Lines         1609      1610    +1     
=========================================
+ Hits          1609      1610    +1     

Impacted Files	Coverage Δ
...penapi_python_client/parser/properties/__init__.py	100.00% <ø> (ø)
...i_python_client/parser/properties/enum_property.py	100.00% <100.00%> (ø)
...on_client/schema/openapi_schema_pydantic/schema.py	100.00% <100.00%> (ø)

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