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/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
index fd6f9fb..ed70a01 100644
--- a/qapi/qapi-dealloc-visitor.c
+++ b/qapi/qapi-dealloc-visitor.c
@@ -103,8 +103,12 @@
     }
 }
 
-static void qapi_dealloc_type_null(Visitor *v, const char *name, Error **errp)
+static void qapi_dealloc_type_null(Visitor *v, const char *name,
+                                   QNull **obj, Error **errp)
 {
+    if (obj) {
+        QDECREF(*obj);
+    }
 }
 
 static void qapi_dealloc_free(Visitor *v)