Fix #915: handle UNLICENSED in NuGet license expressions

[jainlakshya]

Fixes #915

NuGet license expressions can legally include the "UNLICENSED" token. Today, when a package uses that token, cyclonedx-dotnet emits it as a CycloneDX "license.id", which results in invalid output because "UNLICENSED" is not an SPDX license identifier.

Changes:

• When parsing "LicenseType.Expression", map "UNLICENSED" (case-insensitive) to "license.name" instead of "license.id".
• Added a unit test covering the "UNLICENSED" license-expression behavior.

Notes:

• No behavior change for valid SPDX identifiers (e.g., Apache-2.0, MPL-2.0).
• Keeps output valid while still preserving the package’s intent.

Verification:

• dotnet test -c Release
• dotnet format --verify-no-changes --include CycloneDX/Services/NugetV3Service.cs CycloneDX.Tests/NugetV3ServiceTests.cs