If the environment variable $HOME does not exist, don't attempt to
obtain it from /etc/passwd; without $HOME, the calling process
indicates that it does not wish MPD to access the home directory.

This also prevents MPD from attempting to load
`/root/.config/mpd/mpd.conf` if MPD got started as global systemd
service.  Reading from there makes no sense, only /etc/mpd.conf shall
be used then.

This piece of code was initially added by commit 5d857921.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1687