Fix a mistake in ReactChildren refactor (#18380)

* Regression test for map() returning an array

* Add forgotten argument

This fixes the bug.

* Remove unused arg and retval

These aren't directly observable. The arg wasn't used, it's accidental and I forgot to remove. The retval was triggering a codepath that was unnecessary (pushing to array) so I removed that too.

* Flowify ReactChildren

* Tighten up types

* Rename getComponentKey to getElementKey