emacs: Unbreak notmuch-show-rewind (the function to which Backspace is bound)

Commit 095a02211e broke the backspace
key by trying to execute nil as a function, (which obviously won't
work), when it was intended as a return value. Fix this now, (and pine
for a test suite that exercises the emacs user-interface of notmuch).

notmuch.el

@@ -592,7 +592,7 @@ Returns nil if already on the first message in the buffer."
 	      nil
 	    t))
       (recenter 0)
-      (nil))))
+      nil)))
 
 (defun notmuch-show-previous-message ()
   "Backup to the previous message (whether open or closed)