parseProxmoxOnBoot returned nil when the onboot key was absent from a
valid config, but Proxmox's default is onboot=0 (do not start on boot).
This caused stopped VMs without an explicit onboot line (like
windows-runner and tails-anon) to still generate powered-off alerts
despite not being configured to autostart.
Now returns false for absent keys in non-empty configs. nil is reserved
for empty/failed config fetches and unrecognised values, preserving the
'unknown' fallback for genuine uncertainty.
rcourtman
adf6c6f1e5