qapi: Use QNull for a more regular visit_type_null()

Make visit_type_null() take an @obj argument like its buddies.  This
helps keep the next commit simple.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>

include/qapi/visitor-impl.h

@@ -103,7 +103,8 @@ struct Visitor
                      Error **errp);
 
     /* Must be set to visit explicit null values.  */
-    void (*type_null)(Visitor *v, const char *name, Error **errp);
+    void (*type_null)(Visitor *v, const char *name, QNull **obj,
+                      Error **errp);
 
     /* Must be set for input visitors to visit structs, optional otherwise.
        The core takes care of the return type in the public interface. */