bugfix: Course static_tabs importing with display_name = "None"

There is special code in `common/lib/xmodule/xmodule/modulestore/mongo/base.py` to handle setting of `tab['name']` in the course module, for `static_tab` modules which are imported.  This code can overwrite a perfectly fine `tab['name']` value with a `None` during a course import, if the metadata for the static tab module doesn't specify the `display_name`.  

This PR fixes that bug.  Test included.

(it may be that the hack in there should go away entirely: https://github.com/edx/edx-platform/blob/493d8d26e592400230f51905851a23ccfc634d5b/common/lib/xmodule/xmodule/modulestore/mongo/base.py#L679)