add none placeholder for action types

assets/javascripts/discourse/templates/components/wizard-custom-action.hbs

@@ -12,7 +12,7 @@
     <h3>{{i18n "admin.wizard.type"}}</h3>
   </div>
   <div class="setting-value">
-    {{combo-box value=action.type content=types}}
+    {{combo-box value=action.type content=types none="admin.wizard.field.type"}}
   </div>
 </div>