Avoid crashes if postMessage data is null

axe-core crashes if another process sends a `window.postMessage` with
data equals to `null`.

In the previous version, because `null` has a type of `'object'`, it
crashed when trying to access the `channelId` on `null`.

lib/core/utils/frame-messenger/message-parser.js

@@ -67,6 +67,7 @@ export function parseMessage(dataString) {
  */
 function isRespondableMessage(postedMessage) {
   return (
+    postedMessage !== null &&
     typeof postedMessage === 'object' &&
     typeof postedMessage.channelId === 'string' &&
     postedMessage.source === getSource()