Glitch when level labels not filled

The combination of issue title + the patch says enough.

However, to be complete, the observed behavior:
- create a new vocabulary & enable level labels (which you cannot fill at that point)
- go to /taxonomy/N/add/term to add the first term to the vocabulary
- get an error : "warning: strlen() expects parameter 1 to be string, array given in /home/roderik/www/6dev/includes/bootstrap.inc on line 774." (and the HS elements displayed in the 'advanced' fieldset are slightly off)

The reason, ultimately:
somewhere in _hierarchical_select_hierarchy_generate(), t() gets called with an undefined value (because of the level label being unfilled) -- and $label gets set with a huge array ( which is the output of locale(undef, $langcode) )