Handle missing pdbx_PDB_ins_code in mmCIF parser

[LiudengZhang]

What

Handle missing optional pdbx_PDB_ins_code column when parsing mmCIF files.

Why

get_structure() unconditionally accesses atom_site["pdbx_PDB_ins_code"], raising a KeyError for structures that lack this optional column (e.g., IHM entries like 8ZZ4, 9A0P, 9A8I).

How

Added a conditional check: if pdbx_PDB_ins_code exists in atom_site, use it as before; otherwise, fill the ins_code annotation with empty strings (the default value).

Testing

Added a test that removes pdbx_PDB_ins_code from a CIF file's atom_site category and verifies get_structure() succeeds with all-empty insertion codes.

Closes #869

[padix-key]

Looks good, thanks for the fix. This is also a good template for fixing the other optional fields in _atom_site that are still required as mandatory.

[codspeed-hq]

Merging this PR will not alter performance

✅ 63 untouched benchmarks

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_{Comparing LiudengZhang:fix/mmcif-optional-ins-code (9309023) with main (b777f30)}

[padix-key]

The failing test in biotite/biotite/tests/structure/test_info.py::test_one_letter_code is unrelated and likely originates from a new version of the CCD released by the RCSB (will be fixed in #878). I would merge this PR if you agree.

[LiudengZhang]

Thanks for the review! Yes, please go ahead and merge. Happy to help — and good to know this can serve as a template for the other optional fields in _atom_site.