don't generate queries for empty dependencies

--- a/src/HOL/TPTP/mash_export.ML	Fri Aug 03 17:56:35 2012 +0200
+++ b/src/HOL/TPTP/mash_export.ML	Fri Aug 03 17:56:35 2012 +0200
@@ -152,7 +152,8 @@
         val core =
           escape_meta name ^ ": " ^ escape_metas prevs ^ "; " ^
           escape_metas feats
-        val query = if kind <> "" then "? " ^ core ^ "\n" else ""
+        val query =
+          if kind = "" orelse null deps then "" else "? " ^ core ^ "\n"
         val update = "! " ^ core ^ "; " ^ escape_metas deps ^ "\n"
         val _ = File.append path (query ^ update)
       in [name] end