improve register endpoint

api.php

                 $passwordColumnName = $this->getProperty('passwordColumn', 'password');
                 $passwordColumn = $table->getColumn($passwordColumnName);
                 $registerUser = $this->getProperty('registerUser', '');
-                if ($path == 'register' && $registerUser) {
+                if ($path == 'register') {
+                    if (!$registerUser) {
+                        return $this->responder->error(ErrorCode::AUTHENTICATION_FAILED, $username);
+                    }
                     $data = json_decode($registerUser, true);
                     $data = is_array($data) ? $data : [];
                     $data[$usernameColumnName] = $username;