To reproduce:
- enable the block Tag this for a node type
- go view a node that you haven't tagged yet (in my case, other users had tagged it)
- enter a tag and click Add
- the list of all tags refreshes using AJAX, but you can see both your new tag and an empty tag.

I fixed this issue in my installation by editing community_tags.js, in the function Drupal.behaviors.communityTags :

      var tags = $('input[name=tags_refs]', this.form).val().split(', ');
      if (tags.length == 1 && tags[0] == "") tags = []; //added this line to avoid creating empty tags