Handle mailchimp audience not returning a list id (#133)

src/Fieldtypes/MailchimpAudience.php

@@ -21,10 +21,14 @@ protected function toItemArray($id)
         if (! $list = $this->callApi("lists/{$id}")) {
             return [];
         }
-
+
+        if (! $id = Arr::get($list, 'id')) {
+            return [];
+        }
+
         return [
-            'id' => $list['id'],
-            'title' => $list['name'],
+            'id' => $id,
+            'title' => Arr::get($list, 'name'),
         ];
     }
 }