(core) Fix filtering of empty reflists

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

app/common/ColumnFilterFunc.ts

@@ -35,7 +35,11 @@ export function makeFilterFunc(state: FilterState,
   return (val: CellValue) => {
     if (isList(val) && columnType && isListType(columnType)) {
       const list = decodeObject(val) as unknown[];
-      return list.some(item => values.has(item as any) === include);
+      if (list.length) {
+        return list.some(item => values.has(item as any) === include);
+      }
+      // If the list is empty, filter instead by an empty value for the whole list
+      val = columnType === "ChoiceList" ? "" : null;
     }
     return (values.has(Array.isArray(val) ? JSON.stringify(val) : val) === include);
   };