align to latest csstree changes

Author: lahmatiy

lib/compressor/clean/Atrule.js

@@ -13,7 +13,7 @@ module.exports = function cleanAtrule(node, item, list) {
 
     switch (node.name) {
         case 'charset':
-            if (node.expression.children.isEmpty()) {
+            if (!node.expression || node.expression.children.isEmpty()) {
                 list.remove(item);
                 return;
             }

lib/compressor/restructure/2-mergeAtrule.js

@@ -16,7 +16,9 @@ function processAtrule(node, item, list) {
     }
 
     // merge @media with same query
-    if (node.expression.id === prev.expression.id) {
+    if (node.expression &&
+        prev.expression &&
+        node.expression.id === prev.expression.id) {
         prev.block.children.appendList(node.block.children);
         prev.loc = {
             primary: prev.loc,