Avoid null fields

As shown in https://github.com/CycloneDX/cyclonedx-rust-cargo/pull/695 and https://github.com/CycloneDX/cyclonedx-rust-cargo/pull/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 :sweat_smile:. 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` type.

Review the snapshot files and the existing tests to identify where null objects are currently produced, using pull requests 695 and 694 as context. Then inspect the cyclonedx-bom-macros crates and the optional fields involved. Done means a test rejects null objects in snapshots and the proposed attribute macro handles Option fields automatically.

Written by the indexing model from the issue text.