Don't use null for unset()

Revert cs-fixer changes.  Fixed mb-wrapper config

src/Stream/MessagePartStream.php

@@ -62,7 +62,7 @@ public function update(SplSubject $subject) : void
     {
         if ($this->appendStream !== null) {
             // unset forces recreation in StreamDecoratorTrait with a call to __get
-            $this->stream = null;
+            unset($this->stream);
             $this->appendStream = null;
         }
     }