nixos/filesystems: require fstab options list be non-empty

When the option list is empty, the fstab generator does not
automatically add "defaults" and generates a non-working fstab (since it
just emits two spaces around where the options would have been which is
only technically one fstab separator).

nixos/modules/tasks/filesystems.nix

@@ -54,7 +54,7 @@ let
         default = [ "defaults" ];
         example = [ "data=journal" ];
         description = lib.mdDoc "Options used to mount the file system.";
-        type = types.listOf nonEmptyStr;
+        type = types.nonEmptyListOf nonEmptyStr;
       };
 
       depends = mkOption {