Revert "fix(radiosfield, selectfield): allow unset default value"

This reverts commit 020bd45.

Author: btry

inc/field/radiosfield.class.php

@@ -121,7 +121,7 @@ public function prepareQuestionInputForSave($input) {
       $input['values'] = $this->trimValue($input['values']);
       $input['default_values'] = trim($input['default_values']);
 
-      if ($input['default_values'] != '' && !$this->isValidValue($input['default_values'])) {
+      if (!$this->isValidValue($input['default_values'])) {
          Session::addMessageAfterRedirect(
             __('The default value is invalid:', 'formcreator') . ' ' . $input['name'],
             false,