FIX: prevents exception when opening new topic on mobile

Repro:

- Visit https://meta.discourse.org/categories, refresh the page
- Navigate to Latest
- Click New Topic
- Error occurs

app/assets/javascripts/discourse/mixins/open-composer.js.es6

@@ -7,8 +7,8 @@ export default Ember.Mixin.create({
     this.controllerFor('composer').open({
       categoryId: controller.get('category.id'),
       action: Composer.CREATE_TOPIC,
-      draftKey: controller.get('model.draft_key'),
+      draftKey: controller.get('model.draft_key') || Composer.CREATE_TOPIC,
-      draftSequence: controller.get('model.draft_sequence')
+      draftSequence: controller.get('model.draft_sequence') || 0
     });
   },