bug(terraform): Trivy panics when an ignore marker value is unknown

[nikpivkin]

Trivy panics when a value used as an ignore marker is either null or unknown.
Before performing any operations on such a value, we should validate that it is both non-null and known. If the value cannot be resolved at evaluation time, the rule should safely skip it instead of causing a panic.

Steps to reproduce

1. Use the following Terraform configuration:

    #trivy:ignore:*[bucket=mybucket-bucket1]
    resource "aws_s3_bucket" "test" {
      bucket = "mybucket-${each.key}"
    }

2. Run Trivy against this file.

Because the value used as the ignore marker (mybucket-${each.key}) is unknown during evaluation, Trivy attempts to process it and panics instead of handling it gracefully.

Discussed in #9601