feat(data-grid): bulk-edit empty input sets selected cells to NULL

In the 'Bulk Edit Selection' dialog, submitting with an empty input now
sets every selected editable cell to SQL NULL instead of an empty string,
matching the field's 'Value, or NULL' placeholder. fillSelectionWithValue()
accepts string | null so null flows through to applyCellValue().

Co-authored-by: kkk000111999 <kkk000111999@users.noreply.github.com>

apps/desktop/src/components/grid/DataGrid.vue

@@ -4944,7 +4944,7 @@ function applyVisibleCellValue(item: RowItem, visibleCol: number, value: string
   return true;
 }
 
-function fillSelectionWithValue(value: string): boolean {
+function fillSelectionWithValue(value: string | null): boolean {
   const range = selectedRange.value;
   let applied = false;
   if (range) {
@@ -5003,7 +5003,9 @@ function openBulkEditDialog() {
 }
 
 function applyBulkEditValue() {
-  if (!fillSelectionWithValue(bulkEditValue.value)) return;
+  // Empty input sets the selected cells to SQL NULL (the placeholder hints "Value, or NULL").
+  const value = bulkEditValue.value === "" ? null : bulkEditValue.value;
+  if (!fillSelectionWithValue(value)) return;
   bulkEditDialogOpen.value = false;
 }