Handle VS's crazy behaviour of calling DocumentClosing with null.

This seems to happen when closing a solution; so manually respond to
that event by wiping out all documents (closing solution closes all
documents).
Don't crash on project types we can't figure out a path for.

Some return null, some crash; there doesn't seem to be a nice standard
way to get the path for all project types. We might need to handle
additional types here in future, but for now, we'll just skip over those
we don't recognise.