[11.x] Don't touch BelongsTo relationship when it doesn't exist

Currently, MorphTo and BelongsToMany check for relation existence before sending the update query to the database.

This adds that check to the BelongsTo relationship.

`getParentKey()` can be used to find the child foreign key value

Author: patrickomeara

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

@@ -175,6 +175,18 @@ public function match(array $models, Collection $results, $relation)
 
         return $models;
     }
+    
+    /**
+     * Touch all of the related models for the relationship.
+     *
+     * @return void
+     */
+    public function touch()
+    {
+        if (! is_null($this->getParentKey())) {
+            parent::touch();
+        }
+    }
 
     /**
      * Associate the model instance to the given parent.

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

@@ -263,7 +263,7 @@ public function dissociate()
      */
     public function touch()
     {
-        if (! is_null($this->child->{$this->foreignKey})) {
+        if (! is_null($this->getParentKey())) {
             parent::touch();
         }
     }