key group does not default to null

[JamesCalleja]

Describe the Bug

key group defaults to an empty list and this does not allow conditional use

variable "trusted_key_groups" {
type = list(string)
default = []
description = "A list of key group IDs that CloudFront can use to validate signed URLs or signed cookies."
}

Expected Behavior

should be allowed to pass an empty sting and default to null value

Steps to Reproduce

{ zone_id = "XXXXXXXXXXx"
domain_name = "domain-name.com"
bucket = "XXXXXX.s3.amazonaws.com"
trusted_key_groups = ""
comment = "CloudFront"
},

Screenshots

No response

Environment

No response

Additional Context

to fix please update variables

variable "trusted_key_groups" {
type = list(string)
default = null
description = "A list of key group IDs that CloudFront can use to validate signed URLs or signed cookies."
}