[python-package] [ci] fix mypy errors in Booster.__inner_predict()

[jameslamb]

Contributes to #3867.

Fixes the following errors mypy:

python-package/lightgbm/basic.py:4517: error: No overload variant of "__setitem__" of "list" matches argument types "int", "ndarray[Any, dtype[floating[_64Bit]]]"  [call-overload]
python-package/lightgbm/basic.py:4517: note: Possible overload variants:
python-package/lightgbm/basic.py:4517: note:     def __setitem__(self, SupportsIndex, None, /) -> None
python-package/lightgbm/basic.py:4517: note:     def __setitem__(self, slice, Iterable[None], /) -> None
python-package/lightgbm/basic.py:4521: error: "None" has no attribute "ctypes"  [attr-defined]
python-package/lightgbm/basic.py:4527: error: Argument 1 to "len" has incompatible type "None"; expected "Sized"  [arg-type]
python-package/lightgbm/basic.py:4532: error: "None" has no attribute "size"  [attr-defined]
python-package/lightgbm/basic.py:4533: error: "None" has no attribute "reshape"  [attr-defined]
python-package/lightgbm/basic.py:4534: error: Incompatible return value type (got "Optional[Any]", expected "ndarray[Any, Any]")  [return-value]

[github-actions]

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