Summary: When the `getSummarySourceGroup` function (used by the `$group` column) finds that the group is empty, raise a new special exception `EmptySummaryRow`. The engine catches this exception, avoids saving a value to the cell, and removes the record.
Test Plan: Updated several Python tests
Reviewers: georgegevoian
Reviewed By: georgegevoian
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D3489