buffer: increase coverage by removing dead code

buffer.js:L196 `if (value == null)` guarantees `obj != null`
so L406+L418 are unnecessary.

PR-URL: https://github.com/nodejs/node/pull/15100


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>