segmentEach callback args typed as nullable

[graue]

The TypeScript types for segmentEach in @turf/meta have all the arguments to the callback as nullable, but it doesn't appear from either the docs or the code that they can ever be null. This makes it necessary to write pointless checks for undefined to get my code to typecheck.

Using Turf 7.1.0; this is still the case in latest master.