User cannot remove all of their tags from a node

When using community tags with javascript disabled, a user cannot remove all of their tags from a node.

To reproduce:
1. Disable javascript
-- At this point you should see a simple form field for entering or modifying your set of tags.
2. Add one or more tags to a node
-- At this point you can modify the submitted tags, or if multiple tags were entered some tags may be removed.
3. Remove all tags by clearing the form.
-- At this point an error is returned to the user stating "You must enter at least one tag".