emacs: send notmuch-query stderr to /dev/null

The call-process to notmuch in notmuch-query.el was previously sending
stderr into the output buffer.  This means that if there is any stderr
the JSON parsing breaks.  Unfortunately call-process does not support
sending stderr to a separate buffer or to the minibuffer [0], but it
does support sending it to /dev/null.  So we do that here instead.

[0] a bug was filed against emacs (#7842)