fix(misconf): ensure value used as ignore marker is non-null and known

[nikpivkin]

Description

This PR prevents Trivy from panicking when a value used as an ignore marker is null or unknown.

Related issues

• Close bug(terraform): Trivy panics when an ignore marker value is unknown #9834

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).