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>
diff --git a/qapi/trace-events b/qapi/trace-events
index 3b57aba..9e9008a 100644
--- a/qapi/trace-events
+++ b/qapi/trace-events
@@ -31,4 +31,4 @@
 visit_type_str(void *v, const char *name, char **obj) "v=%p name=%s obj=%p"
 visit_type_number(void *v, const char *name, double *obj) "v=%p name=%s obj=%p"
 visit_type_any(void *v, const char *name, void *obj) "v=%p name=%s obj=%p"
-visit_type_null(void *v, const char *name) "v=%p name=%s"
+visit_type_null(void *v, const char *name, void *obj) "v=%p name=%s obj=%p"