fix output of rules with no declarations for Identity compiler

Author: tj

lib/identity.js

@@ -155,6 +155,7 @@ Compiler.prototype.page = function(node){
 Compiler.prototype.rule = function(node){
   var indent = this.indent();
   var decls = node.declarations;
+  if (!decls.length) return '';
 
   return node.selectors.map(function(s){ return indent + s }).join(',\n')
     + ' {\n'