DEV: Prevent empty admin topic buttons from rendering

When using api.decorateWidget("topic-admin-menu:adminMenuButtons") in plugins, an empty button is added if the helper only returns attributes based on a condition (for example, if the admin action is limited to public topics.) In that case, we need to exclude the button from rendering.

app/assets/javascripts/discourse/widgets/topic-admin-menu.js.es6

@@ -272,6 +272,7 @@ export default createWidget("topic-admin-menu", {
         "ul",
         buttons
           .concat(extraButtons)
+          .filter(Boolean)
           .map(b => this.attach("admin-menu-button", b))
       )
     ];