PLANET-7596 Gravity forms won't send thank you pages (#2361)

Ref: https://jira.greenpeace.org/browse/PLANET-7596

- Fixed return type for early exit from string to array

src/GravityFormsExtensions.php

@@ -500,7 +500,7 @@ public function p4_gf_stripe_custom_success_event($entry, $form): array
             !isset($entry['payment_status'])
             || !in_array($entry['payment_status'], $allowed)
         ) {
-            return '';
+            return [];
         }
 
         $tType = [