Fixed project-config/apply error when gateway orderCondition is removed

Added array check for `$event->newValue` in `handleChangedGateway()` to handle cases where nested config keys are removed, which can cause the event to fire with non-array data.

This matches the pattern used in `craft\services\Fields::handleChangedField()`.

Fixes #4185

Author: lukeholder

src/services/Gateways.php

@@ -295,6 +295,11 @@ public function handleChangedGateway(ConfigEvent $event): void
         $gatewayUid = $event->tokenMatches[0];
         $data = $event->newValue;
 
+        // Bail if the data is not a valid gateway config array
+        if (!is_array($data)) {
+            return;
+        }
+
         $transaction = Craft::$app->getDb()->beginTransaction();
         try {
             $gatewayRecord = $this->_getGatewayRecord($gatewayUid);