[dask] fix typehint on _pad_eval_names() (#4413)

python-package/lightgbm/dask.py

@@ -89,7 +89,7 @@ def _remove_list_padding(*args: Any) -> List[List[Any]]:
     return [[z for z in arg if z is not None] for arg in args]
 
 
-def _pad_eval_names(lgbm_model: LGBMModel, required_names: Optional[List[str]] = None) -> LGBMModel:
+def _pad_eval_names(lgbm_model: LGBMModel, required_names: List[str]) -> LGBMModel:
     """Append missing (key, value) pairs to a LightGBM model's evals_result_ and best_score_ OrderedDict attrs based on a set of required eval_set names.
 
     Allows users to rely on expected eval_set names being present when fitting DaskLGBM estimators with ``eval_set``.