Emit strings for CycloneDX metadata property values

[0Cymantek0]

Fixes #5349

metadata.properties values in the CycloneDX JSON output were not strings. errors and warnings were JSON arrays, message was null and system_environment was a nested object. The CycloneDX specification types property values as strings in all versions including the declared 1.3, so schema-validating tools such as cyclonedx-cli reject the output of every scan.

Changes:

• Convert property values to strings in from_headers() with a get_property_string_value() helper. Lists are joined with newlines, mappings are serialized to JSON. One property per list item would work too, joining keeps the property set unchanged.
• Drop empty and null values. The to_dict() filter was meant to remove those but a property mapping is never falsy.
• Update the from_headers test to expect strings and add tests: property values are always strings, empty values are dropped, and an end-to-end check on a scan output.

Validation:

• pytest -vvs tests/formattedcode/test_output_cyclonedx.py : 17 passed, the four new and updated tests fail on develop without the fix
• pytest -q tests/formattedcode/ : 42 passed, 4 skipped
• cyclonedx-python-lib validator: BOMs rejected before the fix with "[] is not of type 'string'", no metadata violations after

Components still fail strict validation because fields such as author and copyright are emitted as null. Separate defect, will be filed separately.

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)