Add indicator to show no stock in part listing

InvenTree/part/templates/part/category.html

@@ -173,7 +173,7 @@
                         return renderLink(value, row.url + 'stock/');
                     }
                     else {
-                        return '';
+                        return "<span class='label label-warning'>No stock</span>";
                     }
                 }
             }