[TwigBridge] Provide a default non-empty label on empty options to validate HTML5

Author: DocFX

Resources/views/Form/form_div_layout.html.twig

@@ -61,7 +61,7 @@
     {%- endif -%}
     <select {{ block('widget_attributes') }}{% if multiple %} multiple="multiple"{% endif %}>
         {%- if placeholder is not none -%}
-            <option value=""{% if required and value is empty %} selected="selected"{% endif %}>{{ placeholder != '' ? (translation_domain is same as(false) ? placeholder : placeholder|trans({}, translation_domain)) }}</option>
+            <option value=""{% if required and value is empty %} selected="selected"{% endif %}>{{ placeholder != '' ? (translation_domain is same as(false) ? placeholder : placeholder|trans({}, translation_domain)) : '--' }}</option>
         {%- endif -%}
         {%- if preferred_choices|length > 0 -%}
             {% set options = preferred_choices %}