process: refactor emitWarning

Remove a couple of obsolete checks by refactoring the code with else.
Also remove the possibility of an undefined warning. That is neither
documented nor does it result in a usable warning.

PR-URL: https://github.com/nodejs/node/pull/20726


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>