FIX: hides topic-footer-mobile-dropdown when no content (#6985)

FIX: hides topic-footer-mobile-dropdown when no content (#6985)

diff --git a/app/assets/javascripts/select-kit/components/topic-footer-mobile-dropdown.js.es6 b/app/assets/javascripts/select-kit/components/topic-footer-mobile-dropdown.js.es6
index 96edaa6..178caa4 100644
--- a/app/assets/javascripts/select-kit/components/topic-footer-mobile-dropdown.js.es6
+++ b/app/assets/javascripts/select-kit/components/topic-footer-mobile-dropdown.js.es6
@@ -8,6 +8,7 @@ export default ComboBoxComponent.extend({
   allowInitialValueMutation: false,
   allowAutoSelectFirst: false,
   nameProperty: "label",
+  isHidden: Ember.computed.empty("content"),
 
   computeHeaderContent() {
     const content = this._super(...arguments);

GitHub sha: c0ccf219