[dask] fix typehint on _pad_eval_names()

[jameslamb]

Following up on #4101 (review) (cc @ffineis just so you know this is what I was talking about in that comment).

The function lightgbm.dask._pad_eval_names() currently has a type hint suggesting that argument required_names could be None, but its code doesn't handle that case. The for loop below would fail with an error complaining about NoneType not being iterable.

LightGBM/python-package/lightgbm/dask.py

Line 98 in b5502d1

for eval_name in required_names:

I don't think it is actually possible for that argument to ever be None, because of

LightGBM/python-package/lightgbm/dask.py

Lines 186 to 189 in b5502d1

	if eval_names:
	evals_result_name = eval_names[i]
	else:
	evals_result_name = f'valid_{i}'

LightGBM/python-package/lightgbm/dask.py

Line 279 in b5502d1

model = _pad_eval_names(model, required_names=evals_result_names)

This PR proposes updating the type hint.

Notes for Reviewers

Noting that in the future, this is the type of thing that will be caught by mypy (#3867). Running mypy --ignore-missing-import python-package/lightgbm yields the following.

python-package/lightgbm/dask.py:98: error: Item "None" of "Optional[List[str]]" has no attribute "iter" (not iterable)

[StrikerRUS]

Good eye!

[ffineis]

Slick, thanks very much for the fix.

[github-actions]

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