Fix Pydantic V1 required field has default=None

[dolfinus]

I've faced a difference in result of the code below between Pydantic versions:

from pydantic import BaseModel

class MyModel(BaseModel):
  field: int

from fieldz import fields

field = fields(MyModel)[0]
assert field.default is field.MISSING

It works on Pydantic v2 but fails with assertion on Pydantic v1. This is because v1 returns ModelField(name='a', type=int, required=True) with default=None.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.33%. Comparing base (7bae0f3) to head (627b3e7).

Files with missing lines	Patch %	Lines
src/fieldz/adapters/_pydantic.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
- Coverage   96.52%   96.33%   -0.20%     
==========================================
  Files          13       13              
  Lines         403      409       +6     
==========================================
+ Hits          389      394       +5     
- Misses         14       15       +1     

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