fix(debian): don't include empty licenses for dpkgs

[DmitriyLewen]

Description

Don't include empty licenses in report.

Also this PR fixes problem with invalid CycloneDX files:

Before:

➜  trivy -q image --format cyclonedx mcr.microsoft.com/playwright:v1.50.0-noble -o playwright-sbom.json 

➜  cyclonedx validate --input-format json --fail-on-errors --input-version v1_6 --input-file playwright-sbom.json
Validating JSON BOM...
Validation failed:
Expected 1 matching subschema but found 0
...

After:

➜  ./trivy -q image --format cyclonedx mcr.microsoft.com/playwright:v1.50.0-noble -o playwright-sbom.json
➜  cyclonedx validate --input-format json --fail-on-errors --input-version v1_6 --input-file playwright-sbom.json
Validating JSON BOM...
BOM validated successfully.

Related issues

• Close bug(dpkg): packages may include empty license strings #8621

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).
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• I've included a "before" and "after" example to the description (if the PR is a user interface change).