(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/client/components/BaseView.js

@@ -319,8 +319,13 @@ BaseView.prototype.filterByThisCellValue = function() {
   // ChoiceList and Reflist values get 'flattened' out so we filter by each element within.
   // In any other column type, complex values (even lists) get converted to JSON.
   let filterValues;
-  if (gristTypes.isList(value) && gristTypes.isListType(col.type.peek())) {
+  const colType = col.type.peek();
+  if (gristTypes.isList(value) && gristTypes.isListType(colType)) {
     filterValues = value.slice(1);
+    if (!filterValues.length) {
+      // If the list is empty, filter instead by an empty value for the whole list
+      filterValues = [colType === "ChoiceList" ? "" : null];
+    }
   } else {
     if (Array.isArray(value)) {
       value = JSON.stringify(value);

app/client/ui/ColumnFilterMenu.ts

@@ -496,6 +496,10 @@ function addCountsToMap(valueMap: Map<CellValue, IFilterCount>, rowIds: RowId[],
     // If row contains a list and the column is a Choice List, treat each choice as a separate key
     if (isList(key) && (columnType === 'ChoiceList')) {
       const list = decodeObject(key) as unknown[];
+      if (!list.length) {
+        // If the list is empty, add an item for the whole list, otherwise the row will be missing from filters.
+        addSingleCountToMap(valueMap, '', () => '', () => '', areHiddenRows);
+      }
       for (const item of list) {
         addSingleCountToMap(valueMap, item, () => item, () => item, areHiddenRows);
       }
@@ -505,6 +509,10 @@ function addCountsToMap(valueMap: Map<CellValue, IFilterCount>, rowIds: RowId[],
     // If row contains a Reference List, treat each reference as a separate key
     if (isList(key) && isRefListType(columnType)) {
       const refIds = decodeObject(key) as unknown[];
+      if (!refIds.length) {
+        // If the list is empty, add an item for the whole list, otherwise the row will be missing from filters.
+        addSingleCountToMap(valueMap, null, () => null, () => null, areHiddenRows);
+      }
       const refLabels = labelMapFunc(rowId);
       const displayValues = valueMapFunc(rowId);
       refIds.forEach((id, i) => {

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);
   };