daemon: Don't print warnings when systemd files are empty

This fixes the warning:

```
Jul 17 17:18:49 stef.thewalter.lan cockpitd[29006]: Failed to load '/run/systemd/system/session-10.scope': Key file does not have group 'Unit'
Jul 17 17:18:49 stef.thewalter.lan systemd-coredump[29666]: Process 29006 (cockpitd) dumped core.
```