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>

scripts/qapi2texi.py

@@ -184,7 +184,7 @@ def texi_sections(doc):
             # prefer @b over @strong, so txt doesn't translate it to *Foo:*
             body += '\n\n@b{%s:}\n' % section.name
         text = str(section)
-        if section.name.startswith('Example'):
+        if section.name and section.name.startswith('Example'):
             body += texi_example(text)
         else:
             body += texi_format(text)