MAE-975: Fix settings page error

The eventextras settings page was returning CiviCRM WSOD because it is mandatory in PHP8 for the first parameter in array_fill_key to be an array.

CRM/EventsExtras/Form/Settings.php

@@ -249,7 +249,8 @@ public function setDefaultValues() {
     }
 
     $defaults['eventsextras_payment_processor_selection_default'] =
-      array_fill_keys($defaults['eventsextras_payment_processor_selection_default'], '1');
+      is_array($defaults['eventsextras_payment_processor_selection_default']) ?
+      array_fill_keys($defaults['eventsextras_payment_processor_selection_default'], '1') : NULL;
 
     return $defaults;
   }