haskell-defaults.nix: added empty 'haskellPlatform' attribute to ghcHEADPrefs

Writing generic expressions is easier if this attribute always exists.

svn path=/nixpkgs/trunk/; revision=34459

pkgs/top-level/haskell-defaults.nix

@@ -17,6 +17,7 @@
 
   ghcHEADPrefs =
     self : self.haskellPlatformArgs_future self // {
+      haskellPlatform = null;
       binary = null; # now a core package
     };