Avoid `array_key_exists()` on `NULL`

The previous code might result in the following error:

Uncaught PHP Exception TypeError: "array_key_exists(): Argument #2 ($array) must be of type array, null given" at [...]/civicrm/ext/de-forumzfd-twinglecampaign/CRM/TwingleCampaign/Utils/APIWrapper.php line 134