Allow specifying [] as filesystem option

because you don't need any on vservers
You still get a warning when not setting the value so that you can't
forget it.

svn path=/nixos/branches/fix-style/; revision=14477

boot/boot-stage-1.nix

@@ -148,7 +148,7 @@ rec {
 
     # !!! copy&pasted from upstart-jobs/filesystems.nix.
     mountPoints =
-      if fileSystems == []
+      if fileSystems == null
       then abort "You must specify the fileSystems option!"
       else map (fs: fs.mountPoint) fileSystems;
     devices = map (fs: if fs ? device then fs.device else "/dev/disk/by-label/${fs.label}") fileSystems;

system/options.nix

@@ -303,7 +303,7 @@ in
 
 
   fileSystems = mkOption {
-    default = [];
+    default = null;
     example = [
       { mountPoint = "/";
         device = "/dev/hda1";