rest_proxy_call_sync: bail out if no payload

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