Fix issue with tabs not being populated

In Studio, tabs are [] by default, not None.  The course descriptor regenerates the tabs if they are None, but not if they are [].  This is okay if tabs stay as an empty list when they are passed to the LMS, because the LMS (courseware/tabs.py) accepts an empty list.  But a list with one item (such as when the open ended or notes tab are added in Studio) will break it.

Adding in the [] check ensures that tabs are current.