make left joins really work

src/PersistentData/PersistentDataManager.php

@@ -227,12 +227,16 @@ class PersistentDataManager implements IPersistentDataManager
 
                 next($data);
             } else if (substr($key, 0, strlen($relation . '__')) === $relation . '__') {
+                if ($data[$relation . '__id'] !== null) {
                     $relationType = current($relations);
                     $relationModel = new $relationType();
                     $this->fillWithData($data, $relationModel, $withRelations, $relation);
 
                     $method = 'set' . str_replace('_', '', ucwords($relation, '_'));
                     $model->$method($relationModel);
+                } else {
+                    next($data);
+                }
 
                 next($relations);
             } else {