systemctl: emit warning when we get an invalid process entry from pid1 and continue

Output looks like this:
Invalid process description in GetUnitProcesses reply: cgroup="machine.slice/machine-rawhide.scope/payload/system.slice/systemd-journald.service" pid=638367 command="/usr/lib/systemd/systemd-journald", ignoring: Invalid argument

(cherry picked from commit 107c2051)