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/qapi.py

@@ -204,7 +204,7 @@ class QAPIDoc(object):
         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)

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)

tests/qapi-schema/doc-bad-section.out

@@ -9,5 +9,5 @@ doc symbol=Enum
 The _one_ {and only}
     arg=two
 
-    section=
+    section=None
 @two is undocumented

tests/qapi-schema/doc-good.out

@@ -82,7 +82,7 @@ doc symbol=Enum
 The _one_ {and only}
     arg=two
 
-    section=
+    section=None
 @two is undocumented
 doc symbol=Base
     body=