Don't break when no home directory can be found

[fasmide]

The systemd example provided in the readme cannot run because mbmd refuses to start when it's unable to figure out a home directory.

pi@powermeter:~ $ journalctl -u mbmd.service 
-- Logs begin at Sun 2020-03-22 23:45:11 GMT, end at Mon 2020-03-23 09:37:22 GMT. --
Mar 23 09:37:16 powermeter systemd[1]: Started mbmd.
Mar 23 09:37:16 powermeter mbmd[898]: $HOME is not defined
Mar 23 09:37:16 powermeter systemd[1]: mbmd.service: Main process exited, code=exited, status=1/FAILURE
Mar 23 09:37:16 powermeter systemd[1]: mbmd.service: Failed with result 'exit-code'.
Mar 23 09:37:17 powermeter systemd[1]: mbmd.service: Service RestartSec=100ms expired, scheduling restart.
Mar 23 09:37:17 powermeter systemd[1]: mbmd.service: Scheduled restart job, restart counter is at 1.
Mar 23 09:37:17 powermeter systemd[1]: Stopped mbmd.

This pull request adds the home directory if it can be determined, otherwise continues without it.

[andig]

Thanks for catching this one, great PR!