Avoid null fields

[pvdrz]

As shown in #695 and #694. Some of the snapshot files contain null objects that are consequence of forgetting sprinking #[serde(skip_serializing_if = "Option::is_none") in some optional fields.

Considering that I was one of the people that forgot to add the attribute 😅. I think it would be nice if:

• A test verifies that snapshot files never contain null objects.
• This behavior could be derived automatically via the cyclonedx-bom-macros crates. Basically adding a new attribute macro that expands by adding #[serde(skip_serializing_if = "Option::is_none") to all fields with Option<T> type.