Update WhatsAppTemplate.php (#508)

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

Author: Pier-d

src/Messages/Channel/WhatsApp/WhatsAppTemplate.php

@@ -34,9 +34,7 @@ public function toArray(): array
             ]
         ];
 
-        if (!is_null($this->context)) {
-            $returnArray['context'] = $this->context;
-        }
+        $returnArray['context'] = $this->context ?? null;
 
         return array_merge($this->getBaseMessageUniversalOutputArray(), $returnArray);
     }
@@ -50,4 +48,4 @@ public function setLocale($locale): void
     {
         $this->locale = $locale;
     }
-}
+}