src: add null check to GetCategoryGroupEnabled()

The input to this function shouldn't be null, and callers are
not equipped to deal with a nullptr return value. Change the
nullptr return to a CHECK_NOT_NULL(). Also fix the indentation
of the function.

PR-URL: https://github.com/nodejs/node/pull/21545
Fixes: https://github.com/nodejs/node/issues/19991


Reviewed-By: Wyatt Preul <wpreul@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>