Fix @option placeholder name mis-match

Closes #3487595

src/Element/StyleSelector.php

@@ -150,7 +150,7 @@ class StyleSelector extends FormElement {
     // Add the empty option if the field is single value and not required.
     // The value 'none' will be added as a NULL value element.
     if (!$element['#multiple'] && !$element['#required']) {
-      $empty_option = ['none' => t('@option', ['@opt' => $element['#empty_option']])];
+      $empty_option = ['none' => t('@option', ['@option' => $element['#empty_option']])];
       $options = $empty_option + $options;
     }