Make empty magit-log-remove-graph-args never remove --graph

This fix treats an empty list (or nil) `magit-log-remove-graph-args'
as "never-match" so to never remove `--graph' in log commands.

Before this fix:

  ;; Never remove `--graph'.  Ugly.
  (setq magit-log-remove-graph-args (list "something that will never match"))

  ;; Remove `--graph' everywhere.  Confusing nil->"true".
  (setq magit-log-remove-graph-args nil)

Now:

  ;; Never remove `--graph'.
  (setq magit-log-remove-graph-args nil)

  ;; Remove `--graph' everywhere.  Empty string regex matches everything.
  (setq magit-log-remove-graph-args (list ""))

lisp/magit-log.el

@@ -938,9 +938,10 @@ Type \\[magit-reset] to reset `HEAD' to the commit at point.
         (unless (magit-file-tracked-p (car files))
           (setq args (cons "--full-history" args)))
       (setq args (remove "--follow" args)))
-    (when (--any-p (string-match-p
-                    (concat "^" (regexp-opt magit-log-remove-graph-args)) it)
-                   args)
+    (when (and (car magit-log-remove-graph-args)
+               (--any-p (string-match-p
+                         (concat "^" (regexp-opt magit-log-remove-graph-args)) it)
+                        args))
       (setq args (remove "--graph" args)))
     (unless (member "--graph" args)
       (setq args (remove "--color" args)))