Fix deprecation warning from passing null to str_replace

The second argument to `str_replace` should be empty string rather than null. Otherwise you get this error:
```
Deprecated: str_replace(): Passing null to parameter #2 ($replace) of type array|string is deprecated
```

Event/Subscriber/DoctrineORMSubscriber.php

@@ -88,7 +88,7 @@ public function filterEntity(GetFilterConditionEvent $event)
 
             if ($dqlFrom = $event->getQueryBuilder()->getDQLPart('from')) {
                 $rootPart = reset($dqlFrom);
-                $fieldName = \str_replace(\sprintf('%s.', $rootPart->getAlias()), null, $event->getField());
+                $fieldName = \str_replace(\sprintf('%s.', $rootPart->getAlias()), '', $event->getField());
                 $metadata = $queryBuilder->getEntityManager()->getClassMetadata($rootPart->getFrom());
 
                 if (isset($metadata->associationMappings[$fieldName]) && (!$metadata->associationMappings[$fieldName]['isOwningSide'] || $metadata->associationMappings[$fieldName]['type'] === ClassMetadata::MANY_TO_MANY)) {