Fix DetailedPRCurve examples for classification tasks

[ntlind]

Improvements

• Ensure that we're correctly accounting for all types of true negatives when pulling examples for DetailedPrecisionRecallCurve, fixing BUG: Valor Core Counting Datums with other label keys as true negative #739
• Add a check to make sure we don't pass any examples when the count of a PR category is zero, fixing BUG: Valor Core Classification Mislabels Prediction FP, FN, TN for examples. #737
• Add tests to check for these issues
• Confirm that benchmarks are unaffected by change