[FIX] typeof null actually returns 'object'

Fixed an issue where calling `isValue` with null would throw a `TypeError` (Cannot read property then of null)

I know it is missing an integration test but that's like hours I'm trying to reproduce this with some tests and cannot spend more time on it now. Tho I didn't want it to be forgotten so here it is.
It is pretty obvious tho, so in case you merge I'll do another PR to add tests.