luci-base: fix empty reply format in ubus gateway fallback code

The ubus gateway fallback code incorrectly formatted ubus replies
containing no payload data when forwarding them via HTTP, leading
to `TypeError: Unexpected reply data format` errors in at least
the `luci.fs` class when receiving replies without payload.

Fix this issue by ensuring that the result array never contains
a `null` value for the payload, send an one-element array containing
just the status code in this case instead.

Fixes: #6074
Signed-off-by: Jo-Philipp Wich <jo@mein.io>