qapi: Unify representation of doc section without name

We have two representations of sections without a name: the main
section uses name=None, the others name=''.  Standardize on name=None.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20171002141341.24616-8-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
4 files changed