Return NULL if undefined

Throwing an exception is not how it is done elsewhere
in the framework, so replaced that by returning NULL
for consistency.

src/Mailer/Mailer.php

@@ -204,7 +204,7 @@ public function offsetExists($offset)
     public function offsetGet($offset)
     {
         if (!$this->offsetExists($offset)) {
-            throw \Exception();
+            return null;
         }
 
         if (isset($this->{$offset})) {