Prefer nil to (current-time) when either will do

Prefer nil to (current-time) when either will do

* lisp/emacs-lisp/timer.el (timer-event-handler, run-at-time):
* lisp/gnus/gnus-score.el (gnus-score-date):
* lisp/gnus/gnus-search.el (gnus-search-query-parse-date)
(gnus-search-imap-handle-date):
* lisp/gnus/gnus-sum.el (gnus-user-date)
(gnus-summary-create-article):
* lisp/image-dired.el (image-dired-create-thumb-1):
* lisp/image/gravatar.el (gravatar-retrieve)
(gravatar--prune-cache):
* lisp/net/dbus.el (dbus-monitor-handler):
* lisp/net/tramp-gvfs.el (tramp-gvfs-handle-set-file-times):
* lisp/net/tramp-sh.el (tramp-sh-handle-set-file-times):
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-handle-set-file-times):
* test/lisp/autorevert-tests.el (auto-revert--wait-for-revert)
(auto-revert-tests--write-file, auto-revert-test--wait-for):
* test/lisp/net/tramp-tests.el (tramp--test-print-duration):
Prefer nil to (current-time) when either will do, as this avoids
some consing.  Similarly, prefer omitting (current-time) arg
when this is equivalent.