FIX: replaces fake null value by clearable option (#8875)

plugins/poll/assets/javascripts/controllers/poll-ui-builder.js.es6

@@ -87,9 +87,9 @@ export default Controller.extend({
 
   @discourseComputed("site.groups")
   siteGroups(groups) {
-    const values = [{ name: "", value: null }];
-    groups.forEach(g => values.push({ name: g.name, value: g.name }));
-    return values;
+    return groups.map(g => {
+      return { name: g.name, value: g.name };
+    });
   },
 
   @discourseComputed("pollType", "regularPollType")

plugins/poll/assets/javascripts/discourse/templates/modal/poll-ui-builder.hbs

@@ -26,6 +26,7 @@
         <label class="input-group-label">{{i18n 'poll.ui_builder.poll_groups.label'}}</label>
         {{combo-box content=siteGroups
                     value=pollGroups
+                    options=(hash clearable=true)
                     valueAttribute="value"}}
       </div>