Always log all components of Transform3D

[emilk]

What

• Closes Make Transform3D always log all its components #6909
• First steps on the road to Allow archetypes to clear and/or update existing components #3381
• Creates a new issue: Visualize density graph of empty batches differently #7117

Best reviewed ignoring whitespace changes.

The serializers for Transform3D and LeafTransform3D will now always write every component, effectively clearing all previously logged values.

Transform3D uses Option<C> for each component, treating it as a single-element list component list where None is empty. If we ever want to support updating some components using the transform (as outlined in #3381) we would need to turn this into a tristate, i.e. something like NoneOrZeroOrOne<C>.

LeafTransform3D uses Option<Vec<C>> for its components, but has no interface yet for logging an update.

Both Transform3D and LeafTransform3D has a new ::clear() constructor in Rust, that replaced both ::new() and ::default(). This is for added clarity, and to make the transition easier if we ever add an ::update() constructor, as per #3381

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!
• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

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[github-actions]

Deployed docs

Commit	Link
e4f6b37	https://landing-n1nv1i4zl-rerun.vercel.app/docs

[Wumpf]

great sample improvements. lgtm minus typos