provide empty() method on Rust archetype implementations to log empty

[Wumpf]

It's cumbersome right now to log an empty set of things:

rec.log("rects_test/rects", &Boxes2D::new([HalfExtents2D; 0]))?;

this should be instead

rec.log("rects_test/rects", &Boxes2D::empty())?;

[nikolausWest]

C++ and Python too?

[teh-cmc]

@jleibs had an intesresting question:

Is this different from Clear?

The answer is yes for three reasons:

1. clear() has optional recursiveness
2. clear() has the out-of-order / pendingness thing (which I'm not sure I understand what that's expected to do anymore)
3. and finally clear() doesnt stop at a specific archetype: if the entity has 10 archetypes logged, it clears all of them

[Wumpf]

In C++ and Python we have harder to avoid default construction which I think is more suited there for creating empty archetypes 🤔

[emilk]

We should make empty() set all the components to empty lists so that they overwrite everything. We should probably make the default constructor do the same.

• Decide if archetypes logging APIs should log non-provided components as empty-lists  #3341

[emilk]

• Closing in favor of Allow archetypes to clear and/or update existing components #3381