proxy-call: warn on empty payload
The assert was introduced by libsoup3 port f014c22b "Port to libsoup3" but it is possible to have NULL payload on error. This was the case with libsoup2 when message->response_body->data was NULL inside g_memdup.
I've found this issue while trying to fix the tests in libgovirt.
Signed-off-by: Victor Toso victortoso@redhat.com