[12.x] Resolve issue with BelongsToManyRelationship factory (#55608)

* Update BelongsToManyRelationship.php

* cs

Author: jackbayliss

src/Illuminate/Database/Eloquent/Factories/BelongsToManyRelationship.php

@@ -50,9 +50,13 @@ public function __construct($factory, $pivot, $relationship)
      */
     public function createFor(Model $model)
     {
-        $relationship = $model->{$this->relationship}();
+        $factoryInstance = $this->factory instanceof Factory;
 
-        Collection::wrap($this->factory instanceof Factory ? $this->factory->prependState($relationship->getQuery()->pendingAttributes)->create([], $model) : $this->factory)->each(function ($attachable) use ($model) {
+        if ($factoryInstance) {
+            $relationship = $model->{$this->relationship}();
+        }
+
+        Collection::wrap($factoryInstance ? $this->factory->prependState($relationship->getQuery()->pendingAttributes)->create([], $model) : $this->factory)->each(function ($attachable) use ($model) {
             $model->{$this->relationship}()->attach(
                 $attachable,
                 is_callable($this->pivot) ? call_user_func($this->pivot, $model) : $this->pivot