[python-package] fix mypy error about pyarrow data

[jameslamb]

Contributes to #3756.
Contributes to #3867.

Fixes the following error from mypy:

python-package/lightgbm/basic.py:2889: error: Incompatible types in assignment (expression has type "list[float] | list[int] | ndarray[Any, Any] | Any | Any | Any | Any | None", variable has type "ndarray[Any, dtype[Any]]")  [assignment]

Notes for Reviewers

For more details on how typing.TypeGuard helps with this, see #6106.

[borchero]

Nice!

Generally, I don't know why but all of the type checkers I've tried misinterpret types used from pyarrow. For example, pyarrow.Table (and all other types fwiw) is interpreted as Any 🙄

[jameslamb]

thanks @borchero !