test: add integration test for domain=null filtering in listCount to fix #27190

Author: aniruddhaadak80

openmetadata-integration-tests/src/test/java/org/openmetadata/it/tests/UserResourceIT.java

@@ -2037,6 +2037,12 @@ void test_listUsersWithDomainFilter(TestNamespace ns) {
     assertTrue(
         findUserInPaginatedResults(user1.getId(), "domain", testDomain().getName()),
         "User in domain should be in filtered list");
+
+    // Also assert that we can search for users without any domain (domain=null)
+    // This specifically tests the issue #27190 where filtering without a domain was throwing a 500 error
+    assertTrue(
+        findUserInPaginatedResults(user2.getId(), "domain", "null"),
+        "User without domain should be in filtered list with domain=null");
   }
 
   @Test