init: disallow "" as out stream;

--- a/src/Pure/Tools/isabelle_process.ML	Thu Sep 04 19:47:13 2008 +0200
+++ b/src/Pure/Tools/isabelle_process.ML	Thu Sep 04 20:05:48 2008 +0200
@@ -100,7 +100,7 @@
 
 fun setup_channels out =
   let val out_stream =
-    if out = "" orelse out = "-" then TextIO.stdOut
+    if out = "-" then TextIO.stdOut
     else
       let
         val path = File.platform_path (Path.explode out);