fix: Omit null fields in CycloneDX SBOM license export

What does this MR do and why?

• The dependency list exports feature fails with SchemaValidationError when exporting CycloneDX SBOMs that have missing optional fields. Although the ingested SBOMs pass CycloneDX validation, GitLab's export fails because null values for optional fields (like license IDs or component versions) are included in the JSON output, violating the CycloneDX schema.

• This MR adds expose_nil: false to the optional fields in the SBOM serialization code:

  • License properties (spdx_identifier, name, url)
  • Component properties (version, purl, licenses)

• In addition, the code now properly validates licenses with the following rules:

  • Valid SPDX ID is the primary validator (even with unknown name) - the license exposure logic was refactored to prioritize SPDX identifiers over names, replacing the previous unknown? method with a valid_id? method that handles all license validation scenarios.
  • license fields with null values are omitted from the output JSON.
  • components with null licenses or empty license arrays skip license serialization.

The following scenarios are now properly handled:

Case	SPDX Identifier	Name	Expected
Standard	MIT	MIT	{ id: "MIT", url: "https://spdx.org/licenses/MIT.html" }
Valid ID, custom name	Apache-2.0	Some License	{ id: "Apache-2.0", url: "https://spdx.org/licenses/Apache-2.0.html" }
Unknown ID and name	unknown	unknown	{ name: "unknown" }
Invalid ID, custom name	nil / "" / unknown	Some License	{ name: "Some License" }
Valid ID, invalid name	Apache-2.0	nil / "" / unknown	{ id: "Apache-2.0", url: "https://spdx.org/licenses/Apache-2.0.html" }

Related issues

• The issue: CycloneDX SBOM export fails validation when pro... (#517492 - closed) • Orin Naaman • 17.11 • At risk
• RFH (internal): https://gitlab.com/gitlab-com/request-for-help/-/issues/2292+s

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