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>
diff --git a/scripts/qapi.py b/scripts/qapi.py
index 2f2738f..2137067 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -204,7 +204,7 @@
         self.section = QAPIDoc.ArgSection(name)
         self.args[name] = self.section
 
-    def _start_section(self, name=''):
+    def _start_section(self, name=None):
         if name in ('Returns', 'Since') and self.has_section(name):
             raise QAPIParseError(self.parser,
                                  "Duplicated '%s' section" % name)