Ensure domain list isn't null before validating

Required for PHP8.1 compatibility.

Closes #3305050

src/Form/SettingsForm.php

@@ -97,7 +97,7 @@ class SettingsForm extends ConfigFormBase {
     foreach (array_keys($checks) as $key) {
       $element = $key . '_domain_list';
       $value = $form_state->getValue($element);
-      if (preg_match('/@+/', $value)) {
+      if ($value && preg_match('/@+/', $value)) {
         $form_state->setErrorByName($element, $this->t('Use domain names only (the part after "@" in an email address)'));
       }
     }