do not error if auto-detect-proxy cmd has no output

do not error if auto-detect-proxy cmd has no output

Regression introduced in 8f858d560e3b7b475c623c4e242d1edce246025a.

Commands are probably better of always having output through as the
fall through to the generic proxy settings is likely not intended. As
documenting and implementing this more consistently is kind of a
regression through, it is split off into the next commit.

Closes: 827713
(cherry picked from commit cad1877559f3e1703c3fea4d081978e1b4bb4a0e)