Allow empty cdrom in proxmox_virtual_environment_vm

[ishioni]

Is your feature request related to a problem? Please describe.
I can't set cdrom file_id = "none" which is the equivalent to a cdrom with no disc inserted in proxmox. When doing this proxmox-side and doing a terraform plan the provider outright shows it's possible,

      ~ cdrom {
          ~ file_id   = "none" -> "ISO:iso/metal-amd64.iso"

But won't take "none" as an argument erroring out.

Describe the solution you'd like
file_id should accept "none" as an argument

Describe alternatives you've considered
I thought I could disable the cdrom outright, but this requires a reboot, whereas removing the disc does not.

Additional context
Add any other context or screenshots about the feature request here.