Default tabs are displayed even when they contain no child elements

Issue Summary

Bam. Title says it all. The patch makes it so that the whole default tab is unset when there aren't any child elements.

In my case, I was using vertical tabs for a content profile that was being displayed during registration. I wanted the extra account fields (email, username) to be in a default tab during registration ("Account info"), but this led to an empty default tab when editing later. This fixed that rare case. I suppose it will also fix when a user doesn't have access to any of the fields in a default tab, or something like that.