fix(python): Improve schema inference for null-first rows

[5vb76]

Closes #21537

Updated the test suite so that running make test now produces the expected output.
Enhance schema‐inference robustness: When infer_schema_length=1 and the first row contains only NULL values, Polars will now scan subsequent rows to determine each column’s actual type—preventing premature failures and improving the user experience.
Test updates: Modified test_infer_schema_length to validate this new, more resilient behavior.

[5vb76]

Hi, reviewers.
I already fix the issue #21537, but all failing checks show I cannot pass the one test named 'test_frame_from_pydecimal_and_ints'. Could u kindly help me with that? I can pass 'make test' in my own repo.