Allow partial dependence plots on trees without validation data

In cases where sklearn uses the fast method for partial dependence plots, no validation data is needed and so ``explain`` can show it without data being passed.

Start at the explain entry point and trace how partial dependence plots select sklearn's fast method versus the path requiring validation data. Confirm the no-data case, then verify that explain can display the plot without validation data and that existing behavior with data remains unchanged.

Written by the indexing model from the issue text.