goa-daemon is crashing on suspend/resume with a traceback that
points here: it calls rest_proxy_call_sync, that calls
_rest_proxy_send_message, assumes it gets a `payload` back,
and calls `finish_call` with it. However, it's not actually
guaranteed that `_rest_proxy_send_message` will return a payload
(a `GBytes`). There are three ways it can return `NULL` instead:
if it's passed a wrong proxy or message, or - when built against
libsoup3 - if there is an error sending the message (it passes
through the return value of `soup_session_send_and_read`, and
that's documented to be `NULL` on error).

If `payload` comes back `NULL`, let's just return `FALSE`, like
we do if there's a problem with the call or message.

Signed-off-by: Adam Williamson <awilliam@redhat.com>