Bugfix: New Tab name being initialized as null

@nedbat @jrbl

STUD-2000 

In Studio, when the course administrator creates a new tab, its name was being
initialized as null in the mongo database. This caused students in LMS to get a
server error when trying to access the course page, since LMS was trying to
process the null tabname for translation, etc. Now tabname is initialized as
'Empty', which is how it was being done in the frontend.
Extra parens around the text are not necessary. I think you misunderstood my comment. Line should read:

display_name = display_name or _("Empty")