Guard against empty result in populate_groupings when getting the counts for a column
If the dashboard is configured to have a column although that column
is never used on any item, then this resulted in a hard crash. [1]
Here we explicitly guard against an empty result and just skip it.
The cells attribute is then not populated with any data (not even zeroes),
but during formatting a zero value is assumed if no cell is found,
so a zero-column will be displayed.
Also add a unit test for populate_groupings confirming the behaviour.
(Interestingly there was a pyright [2] warning on that line,
flagging that data could potentially be None).
Bug: T338684
[1] Stack trace:
File "./property_statistics.py", line 386, in retrieve_data for grouping_item, value in data.items(): AttributeError: 'NoneType' object has no attribute 'items'