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.
It's a nit, but I would globally change all your references to null to None, both here and in the test, and also in your commit message. It'll be slightly easier to read for hardcore python heads.