systemctl: exit with 1 if no unit files found

systemctl: exit with 1 if no unit files found

Add a simple check on the number of unit files that were found: return
`-ENOENT` when none is found from the function and thus `systemctl`
consequently exits with `1` (`EXIT_FAILURE`) if none were found.

Verification:
```bash
root@image:~# systemctl list-unit-files dbus-nonexistant.service; echo
$?
UNIT FILE STATE VENDOR PRESET

0 unit files listed.
1
root@image:~# systemctl list-unit-files dbus.service; echo $?
UNIT FILE    STATE  VENDOR PRESET
dbus.service static enabled

1 unit files listed.
0
```

Fixes #15082.