Use null-device where appropriate

Use null-device where appropriate

* lisp/filesets.el (filesets-select-command):
* lisp/shell.el (shell-mode):
* lisp/term.el (term-exec-1):
* lisp/wdired.el (wdired-do-symlink-changes):
* lisp/cedet/ede/pmake.el (ede-proj-makefile-create):
* lisp/eshell/esh-io.el (eshell-set-output-handle):
* lisp/gnus/gnus-fun.el (gnus-grab-cam-x-face):
* lisp/gnus/mml2015.el (mml2015-epg-key-image):
* lisp/gnus/smime.el (smime-noverify-region):
* lisp/org/ob-picolisp.el (org-babel-execute:picolisp):
* lisp/org/ob-screen.el (org-babel-prep-session:screen)
(org-babel-prep-session:screen):
* lisp/play/fortune.el (fortune-quiet-strfile-options):
* lisp/progmodes/cperl-mode.el (cperl-pod2man-build-command):
* lisp/progmodes/sh-script.el (sh-tmp-file):
* lisp/vc/diff-mode.el (diff-add-log-current-defuns)
(diff--font-lock-prettify):
* lisp/vc/ediff-mult.el (ediff-patch-file-form-meta):
* lisp/vc/ediff-ptch.el (ediff-map-patch-buffer)
(ediff-fixup-patch-map, ediff-dispatch-file-patching-job):
* lisp/vc/vc.el (vc-diff-internal): Use null-device or 	(null-device),
respectively.  (Bug#3736)