Change gift tax rules group validation condition to be more explicit

src/PrestaShopBundle/Form/Admin/Configure/ShopParameters/OrderPreferences/OrderPreferencesFormDataProvider.php

@@ -92,7 +92,7 @@ public function setData(array $data)
         }
 
         // If gift wrapping tax rules group was not submitted - reset it to 0
-        if (!$data['gift_options']['gift_wrapping_tax_rules_group']) {
+        if (!isset($data['gift_options']['gift_wrapping_tax_rules_group'])) {
             $data['gift_options']['gift_wrapping_tax_rules_group'] = 0;
         }