[10.x] Mark model instanse as not exists on deleting MorphPivot relation. (#50135)

* Mark model instanse as not exists on deleting MorphPivot relation.

On deleted event, the model is marked as existing.
Like in AsPivot::delete()

* Update MorphPivot.php

Author: dkulyk

src/Illuminate/Database/Eloquent/Relations/MorphPivot.php

@@ -68,6 +68,8 @@ public function delete()
         $query->where($this->morphType, $this->morphClass);
 
         return tap($query->delete(), function () {
+            $this->exists = false;
+
             $this->fireModelEvent('deleted', false);
         });
     }