fix(table): render empty cells as blank, not em-dash

Empty cells (any column type) showed an em-dash placeholder. Drop it
so empty cells render blank — matches what the user expects when
nothing's there.

Author: TheodoreSpeaks

apps/sim/app/workspace/[workspaceId]/tables/[tableId]/components/table-grid/cells/cell-render.tsx

@@ -232,7 +232,7 @@ export function CellRender({ kind, isEditing }: CellRenderProps): React.ReactEle
       )
 
     case 'empty':
-      return <span className='text-[var(--text-tertiary)]'>—</span>
+      return null
 
     default: {
       // Exhaustiveness guard: TypeScript flags this branch if a new