fix(sbom): scan results of SBOMs generated from container images are missing layers

[fabriziosestito]

Description

When scanning a SBOM generated from a container image, layers are missing from the scan results.
This happens because layers are explicitly removed if the scanned artifact is not of the type container image.
However, this removes the layers when a container image SBOM is generated from a container image.
Moreover, setting layers to an empty value is not needed as layers are not present when scanning artifacts that are not container images or SBOMs of container images anyway.

Related issues

• Related to discussion: Scanning an SBOM doesn't include container layer information, even if the SBOM specifies a type of container #6504

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).

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[knqyf263]

Thanks for your contribution. Could you fix tests?

[fabriziosestito]

@knqyf263 I've updated the existing integration tests and fixtures in 83479df