Add parens around ?? operator

Author: twomice

CRM/Percentagepricesetfield/Form/Settings.php

@@ -178,7 +178,7 @@ private function saveSettings() {
   public function setDefaultValues() {
     $result = civicrm_api3('setting', 'get', array('return' => array_keys($this->_settings)));
     $domainID = CRM_Core_Config::domainID();
-    $ret = $result['values'][$domainID] ?? NULL;
+    $ret = ($result['values'][$domainID] ?? NULL);
     return $ret;
   }