Make sure the flag form element doesn't appear on non-edit forms

Closes #3169627

flag.module

@@ -72,10 +72,9 @@ function flag_form_alter(&$form, FormStateInterface $form_state, $form_id) {
 
   // Check the first flag and return early if the form isn't considered to be
   // an edit form.
-  if (!empty($flags) && isset($flags[0]) && $flags[0] instanceof FlagInterface) {
-    /** @var \Drupal\flag\Plugin\Flag\EntityFlagType $flag_type */
-    $flag_type = $flags[0]->getFlagTypePlugin();
-    if (!$flag_type->isAddEditForm($object->getOperation())) {
+  $first_flag = reset($flags);
+  if ($first_flag instanceof FlagInterface) {
+    if (!$first_flag->getFlagTypePlugin()->isAddEditForm($object->getOperation())) {
       return;
     }
   }