Fix crash when 0 rows of class

[Innixma]

Issue #, if available:

Description of changes:

• When the label column is dtype categorical, and a category exists that is present in 0 rows of the training data, an exception would be raised 'NoneType' object has no attribute 'reset_index'. This PR fixes this issue by returning early in the function call.
• Note that this bug would only appear in very specific situations, and only if the user somehow had their dtype be category for the label column.
• Additionally fixes FutureWarnings from usage of deprecated functions .append and .iteritems

This bug was found while expanding the tabular benchmark suite to 300 datasets (from 100), as it caused 2 datasets to crash.

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[github-actions]

Job PR-2990-c44c1c7 is done.
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