trace_rules: only non-empty;

--- a/src/Pure/Isar/method.ML	Tue Oct 23 19:14:31 2001 +0200
+++ b/src/Pure/Isar/method.ML	Tue Oct 23 19:14:47 2001 +0200
@@ -133,7 +133,7 @@
 val trace_rules = ref false;
 
 fun trace rules =
-  if not (! trace_rules) then ()
+  if not (! trace_rules) orelse null rules then ()
   else Pretty.writeln (Pretty.big_list "rules:" (map Display.pretty_thm rules));