fix mypy errors in firm score

[nicholasloveday]

multicategorical_impl.py:109: error: Argument 5 to "_single_category_score" has incompatible type "float | None"; expected "float" [arg-type]
multicategorical_impl.py:117: error: Item "int" of "Dataset | Literal[0]" has no attribute "mean" [union-attr]
multicategorical_impl.py:210: error: Incompatible types in assignment (expression has type "int", variable has type "ndarray[Any, dtype[Any]]") [assignment]
multicategorical_impl.py:211: error: Incompatible types in assignment (expression has type "int", variable has type "ndarray[Any, dtype[Any]]") [assignment]
Found 4 errors in 1 file (checked 1 source file)