Fix copy-on-write chained assignment for default assignment

[johnchristopherjones]

With Copy-On-Write enabled with at least Pandas 2.2.0, Pandera emits ChainedAssignmentError warnings whenever setting defaults:

[…]/.venv/lib/python3.11/site-packages/pandera/backends/pandas/array.py:337: ChainedAssignmentError: A value is trying to be set on a copy of a DataFrame or Series through chained assignment using an inplace method.
When using the Copy-on-Write mode, such inplace method never works to update the original DataFrame or Series, because the intermediate object on which we are setting values always behaves as a copy.

For example, when doing 'df[col].method(value, inplace=True)', try using 'df.method({col: value}, inplace=True)' instead, to perform the operation inplace on the original object.


  check_obj[schema.name].fillna(schema.default, inplace=True)

This PR just changes the offending line to use the suggested syntax.

[johnchristopherjones]

Resolved by #1464