Serializers may serialize null fields

[iantdunn]

Overview

All serializers may currently serialize null SBOM and component fields to the file. Instead of writing null values, we need to have a good way to check to simply not add those fields to the final SBOM string in the serialize() method.

Resources

• Convert Refactor #261
• Current solution

Acceptance Criteria

• Do not serialize null fields
• ~80% Code Coverage

[dlg1206]

@txdvse May have worked with this for #261