fix: [hr] searchUsers에서 q 파라미터 null 처리

- ConvertEmptyStringsToNull 미들웨어로 인해 빈 q= 가 null로 변환되는 문제

app/Http/Controllers/Api/Admin/HR/EmployeeController.php

@@ -45,7 +45,7 @@ public function index(Request $request): JsonResponse|Response
      */
     public function searchUsers(Request $request): JsonResponse
     {
-        $users = $this->employeeService->searchTenantUsers($request->get('q', ''));
+        $users = $this->employeeService->searchTenantUsers($request->get('q') ?? '');
 
         return response()->json([
             'success' => true,