Image ID Regex - Include "null" to allow missing or computed values

[bdavisOSSG]

Hi,

This does not seem to work .. for a nul value

https://github.com/hashicorp/terraform-sentinel-policies/tree/main/azure/restrict-vm-image-id.sentinel

this is the image details form the workspace state

and it fails the check on

`Print messages:

azurerm_windows_virtual_machine.ansible_wintst has source_image_id with value that does not match the regex ^(/subscription`

[bdavisOSSG]

HI,

the word "nul" does not work,

but

image_id_regex = "^(/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/(.*)/providers/Microsoft.Compute/images/(.*)|)$"

Does .. effectively making the regex ^$ which is nothing ..