Add custom JSON Marshallers functions for CycloneDX vulnerability data structures

[mrutkows]

When outputting data in JSON format, we need to remove all empty nested structs. We have done this for the license command. This reduces JSON output for downstream processing.

See this issue for more background: #18

See what was done for the license command here as part of the fix for issue 18: https://github.com/CycloneDX/sbom-utility/blob/custom-cmds/schema/cyclonedx_marshal.go

[mrutkows]

Partially fixed by: #40