(core) Automatically remove empty summary table rows

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

sandbox/grist/table.py

@@ -536,9 +536,17 @@ class Table(object):
       # the column named self._summary_helper_col_id is a single reference
       # or a reference list.
       lookup_value = rec if self._summary_simple else functions.CONTAINS(rec)
-      return self._summary_source_table.lookup_records(**{
+      result = self._summary_source_table.lookup_records(**{
         self._summary_helper_col_id: lookup_value
       })
+
+      # Remove rows with empty groups
+      self._engine.docmodel.setAutoRemove(rec, not result)
+      if not result:
+        # The group is empty, tell the engine that this record will be deleted
+        raise EmptySummaryRow()
+
+      return result
     else:
       return None
 
@@ -633,3 +641,10 @@ class Table(object):
     # creates a dependency and brings formula columns up-to-date.
     self._engine._use_node(col.node, relation, row_ids)
     return [col.get_cell_value(row_id) for row_id in row_ids]
+
+
+class EmptySummaryRow(Exception):
+  """
+  Special exception indicating that the summary group is empty and the row should be removed.
+  """
+  pass