Add specs for topic create with auto_close_time. Just ignore auto_close_time from users who are not authorized to use it instead of raising an error.

app/assets/javascripts/discourse/lib/utilities.js

@@ -339,7 +339,7 @@ Discourse.Utilities = {
           return t.add('days', 1).toJSON();
         }
       } else {
-        return arg;
+        return (arg === '' ? null : arg);
       }
     }
   }

app/assets/javascripts/discourse/models/composer.js

@@ -443,7 +443,6 @@ Discourse.Composer = Discourse.Model.extend({
         postStream = this.get('topic.postStream'),
         addedToStream = false;
 
-
     // Build the post object
     var createdPost = Discourse.Post.create({
       raw: this.get('reply'),