use subGroupCount instead (keycloak#28719)

fixes: keycloak#28684

Signed-off-by: Erik Jan de Wit <erikjan.dewit@gmail.com>
(cherry picked from commit 03c2629)

js/apps/admin-ui/src/clients/authorization/policy/Group.tsx

@@ -160,7 +160,7 @@ export const Group = () => {
                           name="extendChildren"
                           isChecked={field.value}
                           onChange={field.onChange}
-                          isDisabled={group.subGroups?.length === 0}
+                          isDisabled={group.subGroupCount === 0}
                         />
                       )}
                     />