Last modified: 2014-06-26 00:40:00 UTC
If you add an empty, unnamed reference in wikitext ("<ref />"), then you get an empty, invalid reference. If you add an empty, unnamed reference in VisualEditor (Insert>Reference, then "Add reference" without typing anything), then you get an empty, invalid reference. VisualEditor should do better than wikitext: it should detect empty, unnamed ref tags and refuse to insert them.
We fixed this a while ago. Sorry for the slow triage!