Backport: Don't compare arguments that can be nil (Bug#28039)

copy-region-as-kill can be called passing nil as `beg' and
`end'. Magit does that, which caused an error when this advice was in
effect.

* lisp/ses.el (ses--advice-copy-region-as-kill): avoid comparison
  unless `beg' and `end' are non-nil.

lisp/ses.el

@@ -3051,7 +3051,7 @@ We'll assume copying front-sticky properties doesn't make sense, either.
 
 This advice also includes some SES-specific code because otherwise it's too
 hard to override how mouse-1 works."
-  (when (> beg end)
+  (when (and beg end (> beg end))
     (let ((temp beg))
       (setq beg end
 	    end temp)))