qapi: Disallow qmp_marshal_FOO(NULL, ...)

For QMP commands without arguments, gen_marshal() laboriously
generates a qmp_marshal_FOO() that copes with null @args.  Turns
there's just one caller that passes null instead of an empty QDict.
Adjust that caller, and simplify gen_marshal().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200424084338.26803-15-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
3 files changed