pg/Connection: allow undefined ReceiveResult() return value
pg/Connection: allow undefined ReceiveResult() return value

This happens when the end of the response was reached, and is not an
error.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy