perf: Allow components to have null children

[st-pasha]

Description

The Component.children property can now be null.

The reasoning is simple: most of the components in the game tree are leaf nodes, and don't have any children. For these components carrying around a ComponentSet object is wasteful.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, doc: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples.

Breaking Change

• No, this is not a breaking change.

Related Issues

[spydon]

I think this might be premature optimization (especially without benchmarking), or have you actually faced a problem with this? In some cases it will actually make your game less performant, since it can create the component sets outside of onLoad with this change. I'm okay with having this merged anyways though since the scenario you describe is way more common.