Fix get_subscriber_id definition when no subscribers found by email

Author: n7studios

src/ConvertKit_API_Traits.php

@@ -863,6 +863,10 @@ public function get_subscriber_id(string $email_address)
             ['email_address' => $email_address]
         );
 
+        if (!count($subscribers->subscribers)) {
+            return false;
+        }
+
         // Return the subscriber's ID.
         return $subscribers->subscribers[0]->id;
     }