[SPARK-22106][PYSPARK][SQL] Disable 0-parameter pandas_udf and add doctests

## What changes were proposed in this pull request?

This change disables the use of 0-parameter pandas_udfs due to the API being overly complex and awkward, and can easily be worked around by using an index column as an input argument.  Also added doctests for pandas_udfs which revealed bugs for handling empty partitions and using the pandas_udf decorator.

## How was this patch tested?

Reworked existing 0-parameter test to verify error is raised, added doctest for pandas_udf, added new tests for empty partition and decorator usage.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #19325 from BryanCutler/arrow-pandas_udf-0-param-remove-SPARK-22106.