Fix dropping @media rules with nested not conditions

fixtures/compress/atrules/media-not-nested.css

@@ -0,0 +1,27 @@
+/* issue #476: nested `not` in media conditions must not drop the @media rule */
+@media (max-width: 701px) and (not (any-pointer: fine)) {
+    .a { color: red; }
+}
+
+@media (not (color)) {
+    .b { color: blue; }
+}
+
+@media (max-width: 701px) and (not (any-pointer: fine)), (hover: none) {
+    .c { color: green; }
+}
+
+@media not screen {
+    .d { color: black; }
+}
+
+@media (max-width: 701px) and (not (any-pointer: fine)) {
+    .x { color: red; }
+}
+
+@media (max-width: 701px) and (not (any-pointer: fine)) {
+    .y { color: blue; }
+}
+
+@media (max-width: 701px) and (not (any-pointer: fine)) {
+}

lib/clean/Atrule.js

@@ -1,5 +1,5 @@
 import { keyword as resolveKeyword } from 'css-tree';
-import { hasNoChildren } from './utils.js';
+import { hasNoChildren, isEmptyAtrulePrelude } from './utils.js';
 
 export default function cleanAtrule(node, item, list) {
     if (node.block) {
@@ -16,7 +16,7 @@ export default function cleanAtrule(node, item, list) {
 
     switch (node.name) {
         case 'charset':
-            if (hasNoChildren(node.prelude)) {
+            if (isEmptyAtrulePrelude(node.prelude)) {
                 list.remove(item);
                 return;
             }
@@ -60,7 +60,7 @@ export default function cleanAtrule(node, item, list) {
                 name === 'supports') {
 
                 // drop at-rule with no prelude
-                if (hasNoChildren(node.prelude) || hasNoChildren(node.block)) {
+                if (isEmptyAtrulePrelude(node.prelude) || hasNoChildren(node.block)) {
                     list.remove(item);
                 }
             }

lib/clean/utils.js

@@ -2,6 +2,21 @@ export function hasNoChildren(node) {
     return !node || !node.children || node.children.isEmpty;
 }
 
+// css-tree emits a Raw node (no `children` list) when a prelude fails to parse,
+// e.g. Media Queries Level 4 nested `not` conditions. A non-empty Raw value is
+// still a real prelude and must not be treated as missing (issue #476).
+export function isEmptyAtrulePrelude(node) {
+    if (!node) {
+        return true;
+    }
+
+    if (node.type === 'Raw') {
+        return !node.value;
+    }
+
+    return hasNoChildren(node);
+}
+
 export function isNodeChildrenList(node, list) {
     return node !== null && node.children === list;
 }