[dask] fix mypy errors about padded eval_results

[jameslamb]

Contributes to #3867.

Fixes the following mypy errors.

python-package/lightgbm/dask.py:166: error: Incompatible types in assignment (expression has type "str", target has type "Dict[str, List[Any]]")  [assignment]
python-package/lightgbm/dask.py:168: error: Incompatible types in assignment (expression has type "str", target has type "Dict[str, float]")  [assignment]

[github-actions]

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