confusion_matrix, roc_curve: Use standardized nullable type handling

[tamargrey]

Currently, we handle nullable types for the confusion_matrix and roc_curve with the _convert_ww_series_to_np_array util, which works around the inability of Series.to_numpy to recognize the pandas nullable dtypes, resulting in numpy data with the object dtype, which causes errors down the line.

We should move _convert_ww_series_to_np_array to the nullable type handling utils file and rename it to make it celar it has to do with nullable type handling.