BUG: groupby.idxmax/idxmin consistently raise on unobserved categorical
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Part of #10694, but doesn't close it fully.

 - When given a non-ordered category with unobserved categories, we currently raise about the unobserved categories. We should instead raise about the non-orderedness for consistency with min/max.
 - _python_apply_general does not return the correct dtype when there is a CategoricalIndex will NaN values.
 - _python_apply_general fails on a single grouping with unobserved categories even when we're call it from transform where the unobserved categories should have no impact.
 - _python_apply_general with an empty DataFrame with no numeric columns returns all the columns even when `numeric_only=True`.

All of these are fixed in #54234
Friendly ping @WillAyd @jbrockmendel  @WillAyd - are you good here?