F null pointer list users

Summary

• Test for caosdb-cpplib!47 (merged)

• Also confirms https://gitlab.com/linkahead/linkahead-server/-/issues/250 (https://gitlab.indiscale.com/caosdb/src/caosdb-cppinttest/-/jobs/262345#L2733)

test/test_user.cpp

@@ -66,4 +66,22 @@ TEST(test_user, test_create_single_user_success) {
     "transport or RPC protocol layer. Original error: 5 - This account does not exist.");
 }
 
+TEST(test_user, test_list_users) {
+  auto connection = ConnectionManager::GetDefaultConnection();
+  auto users = connection->ListUsers();
+  EXPECT_EQ(users.size(), 1);
+
+  User user("CaosDB", "user1");
+  user.SetPassword("Password1!");
+  connection->CreateSingleUser(user);
+
+  users = connection->ListUsers();
+  EXPECT_EQ(users.size(), 2);
+
+  connection->DeleteSingleUser("CaosDB", "user1");
+
+  users = connection->ListUsers();
+  EXPECT_EQ(users.size(), 1);
+}
+
 } // namespace caosdb::connection