fix: form field Enum overrides

[rgimen3z]

I noticed when using an optional FormFieldSelect, the default description and placeholder would override the ones I passed to the json_schema_extra.

Fixed using a similar strategy to what we already do a few lines below in https://github.com/pydantic/FastUI/blob/196414360b69b3dab7012576f852229831307883/src/python-fastui/fastui/json_schema.py#L334

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.00%. Comparing base (16abe6a) to head (ac842e4). Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #235      +/-   ##
==========================================
+ Coverage   95.68%   96.00%   +0.32%     
==========================================
  Files          14       14              
  Lines         950      951       +1     
==========================================
+ Hits          909      913       +4     
+ Misses         41       38       -3     

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