kdump.service fails when configuring it for "ssh" and only giving a host name without a `user@` part.

There is no useful error message, so it's unnecessarily hard to see what's wrong.

Reproducible: Always

Steps to Reproduce:
cat <<EOF > /etc/kdump.conf
auto_reset_crashkernel yes
path /var/crash
core_collector makedumpfile -l --message-level 7 -d 31 -F
failure_action reboot
ssh localhost
sshkey /root/.ssh/id_rsa
EOF
systemctl restart kdump
Actual Results:
Job for kdump.service failed because the control process exited with error code.
See "systemctl status kdump.service" and "journalctl -xeu kdump.service" for details.

and journal just says

systemd[1]: Starting kdump.service - Crash recovery kernel arming...
kdumpctl[11597]: Pseudo-terminal will not be allocated because stdin is not a terminal.
kdumpctl[11591]: kdump: Starting kdump: [FAILED]
systemd[1]: kdump.service: Main process exited, code=exited, status=1/FAILURE

Expected Results:
Preferably, allow this, and default to root@, i.e what ssh does. Alternatively, give an useful error message about the configuration being invalid.