[CSSOM] LinkStyle.sheet should be nullable

IDL says:

   interface LinkStyle {
     readonly attribute StyleSheet sheet;
   };

Prose says:

    The sheet attribute must return the associated style sheet for the
    node, or null, if there is no associated style sheet.

which is impossible given the IDL.  I believe the prose is correct as 
far as it goes and the IDL is wrong.

-Boris