fix: include empty values in missing data view filters

src/components/MissingDataView.tsx

@@ -112,7 +112,7 @@ export function MissingDataView({ data, headers, onSaveRow, onCaptureState }: Mi
         } else {
           val = String(val ?? '');
         }
-        if (val) result[col].add(val);
+        result[col].add(val);
       });
     });
     return result;