Avoid setting null when an string is required

Author: ajgarlag

tests/TestHelper.php

@@ -82,7 +82,7 @@ public static function generateJwtToken(AccessTokenModel $accessToken): string
         $accessTokenEntity->setIdentifier($accessToken->getIdentifier());
         $accessTokenEntity->setExpiryDateTime($accessToken->getExpiry());
         $accessTokenEntity->setClient($clientEntity);
-        $accessTokenEntity->setUserIdentifier($accessToken->getUserIdentifier());
+        $accessTokenEntity->setUserIdentifier((string) $accessToken->getUserIdentifier());
 
         foreach ($accessToken->getScopes() as $scope) {
             $scopeEntity = new ScopeEntity();