unified return null usages

Author: fabpot

Tests/Normalizer/GetSetMethodNormalizerTest.php

@@ -144,7 +144,7 @@ public function provideCallbacks()
             array(
                 array(
                     'bar' => function ($bar) {
-                        return null;
+                        return;
                     },
                 ),
                 'baz',