Update WhatsAppAudio.php (#514)

Avoiding Error Typed property Vonage\Messages\Channel\WhatsApp\WhatsAppAudio::$context must not be accessed before initialization

src/Messages/Channel/WhatsApp/WhatsAppAudio.php

@@ -26,10 +26,7 @@ public function toArray(): array
     {
         $returnArray = $this->getBaseMessageUniversalOutputArray();
         $returnArray['audio'] = $this->audioObject->toArray();
-
-        if (!is_null($this->context)) {
-            $returnArray['context'] = $this->context;
-        }
+        $returnArray['context'] = $this->context ?? null;
 
         return $returnArray;
     }