Fix problems with empty component ID registration

Fixes #2493, also closes #2353 as it incorporates a variant of that fix.

- Instead of having getEmptyComponent return `<noscript />` directly, wrap it in a class that we can easily identify later as being an empty component
- Cache the empty component element instead of recreating one each time
- Avoid touching the nullComponentIdsRegistry dictionary at all when not dealing with empty components
- Move empty-component tests to a separate file

Test Plan: jest
Hah, I meant my first bare React component class. I can use `class` if that's preferred.