[TheNetherWatcher]

Found by @tompollard while reviewing #605.

The wrann docstring (and the Annotation example) say an aux note can be None for annotations without a note, but None was rejected:

wfdb.wrann("a", "atr", np.array([1, 5, 9]), symbol=["N", "N", "V"], aux_note=[None, "(VT", None])
# TypeError: Subelements of the aux_note field must be strings

compact_fields already turns empty notes into None before writing, so the writer handles None. Only check_field rejected it. It now skips None in aux_note, so None is written exactly like "" (byte-identical files), and rdann reads both back as "". I kept the docstring as it is, since it already describes this.

Tests: None and "" notes give byte-identical files, and read back as "". The test fails on main. pytest -m "not network" passes (100 tests), and the docs build passes.