Summary:
A formula returning an empty RecordSet in a RefList columns results in storing [] instead of null.
This caused a bug where the empty list was 'flattened' and the cell not appearing in filters at all.
This diff fixes the bug by filtering for the default value `null` instead for RefLists and the empty string for ChoiceLists.
I didn't manage to actually reproduce the bug for ChoiceLists, but this seemed the most sensible thing to do.
Test Plan: New nbrowser test.
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D3478