[5.4] Null Morph Map ErrorException (#18921)

* [5.4] Null Morph Map ErrorException

Currently, if you call `getActualClassNameForMorph` without the morph map set, it will through an error because `Relation::morphMap()` returns `null` instead of (what I would expect) `[]`.

Is this expected behaviour? If so, why is this expected behaviour?

* Create HasRelationships.php

src/Illuminate/Database/Eloquent/Concerns/HasRelationships.php

@@ -192,7 +192,7 @@ protected function morphInstanceTo($target, $name, $type, $id)
      */
     public static function getActualClassNameForMorph($class)
     {
-        return Arr::get(Relation::morphMap(), $class, $class);
+        return Arr::get(Relation::morphMap() ?: [], $class, $class);
     }
 
     /**