Allow relationship to be null

Author: tobyzerner

src/AbstractSerializer.php

@@ -56,8 +56,8 @@ public function getRelationship($model, $name)
         if (method_exists($this, $name)) {
             $relationship = $this->$name($model);
 
-            if (! ($relationship instanceof Relationship)) {
-                throw new LogicException('Relationship method must return an instance of '
+            if ($relationship !== null && ! ($relationship instanceof Relationship)) {
+                throw new LogicException('Relationship method must return null or an instance of '
                     .Relationship::class);
             }