redirect which stderr to /dev/null for consistency

redirect which stderr to /dev/null for consistency

The "standard" which (debianutils) has no output whatsoever on stderr,
bash and dash which use this implementation therefore haven't either.
In zsh 'which' is a shell built-in – and has no stderr output either, it
does print an error message on stdout…

So, realistically, a redirection isn't needed at all, but it also can't
hurt (<- I have said that before in this context ->) so why not for
consistency with… well, not with "command -v" as that hasn't an error
message either. Lets say for consistency with my mental image of shell,
as I am still a bit puzzled by zsh's which and now could imagine even
more strange things in other shells.

Closes: 807373