don't launch the automatic minimizer for zero facts

--- a/src/HOL/Tools/Sledgehammer/sledgehammer_run.ML	Wed Jun 08 16:20:18 2011 +0200
+++ b/src/HOL/Tools/Sledgehammer/sledgehammer_run.ML	Wed Jun 08 16:20:18 2011 +0200
@@ -77,7 +77,7 @@
              ({state, subgoal, subgoal_count, facts, ...} : prover_problem)
              (result as {outcome, used_facts, run_time_in_msecs, preplay,
                          message, message_tail} : prover_result) =
-  if is_some outcome then
+  if is_some outcome orelse null used_facts then
     result
   else
     let