[python-package] fix mypy errors in plotting.py

[jameslamb]

Contributes to #3867.

This PR fixes mypy errors in plotting.py.

python-package/lightgbm/plotting.py:243: error: Value of type "Union[int, str, None]" is not indexable
python-package/lightgbm/plotting.py:243: error: Unsupported left operand type for - ("str")
python-package/lightgbm/plotting.py:243: note: Both left and right operands are unions
python-package/lightgbm/plotting.py:244: error: Unsupported operand types for / ("str" and "int")
python-package/lightgbm/plotting.py:244: note: Left operand is of type "Union[Any, str]"
python-package/lightgbm/plotting.py:244: error: Value of type "Union[int, str, None]" is not indexable
python-package/lightgbm/plotting.py:256: error: Value of type "Union[int, str, None]" is not indexable
python-package/lightgbm/plotting.py:256: error: Unsupported left operand type for - ("str")
python-package/lightgbm/plotting.py:256: note: Both left and right operands are unions
python-package/lightgbm/plotting.py:257: error: Value of type "Union[int, str, None]" is not indexable
python-package/lightgbm/plotting.py:361: error: Incompatible types in assignment (expression has type "Iterator[Any]", variable has type "Optional[List[str]]")
python-package/lightgbm/plotting.py:365: error: Incompatible types in assignment (expression has type "Iterator[str]", variable has type "Optional[List[str]]")
python-package/lightgbm/plotting.py:367: error: No overload variant of "next" matches argument type "Optional[List[str]]"
python-package/lightgbm/plotting.py:367: note: Possible overload variant:
python-package/lightgbm/plotting.py:367: note: def [_T] next(Iterator[_T]) -> _T
python-package/lightgbm/plotting.py:367: note: <1 more non-matching overload not shown>
python-package/lightgbm/plotting.py:384: error: Item "None" of "Optional[List[str]]" has no attribute "iter" (not iterable)

Confirmed that this suppresses these warnings by running the following.

mypy \
    --exclude='python-package/compile/|python-package/build' \
    --ignore-missing-imports \
    python-package/

[StrikerRUS]

LGTM, thank you!

[github-actions]

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