fix: more robust rootShadowHost check (#50)

This _may_ be the underlying cause
[here](https://sentry.sentry.io/issues/3725692470/?project=11276&referrer=assigned_activity-email),
maybe an e.g. `undefined` sneaked in there somehow. IMHO this should
also safe some bytes.

packages/rrweb/src/record/mutation.ts

@@ -282,7 +282,7 @@ export default class MutationBuffer {
       // ensure shadowHost is a Node, or doc.contains will throw an error
       const notInDoc =
         !this.doc.contains(n) &&
-        (rootShadowHost === null || !this.doc.contains(rootShadowHost));
+        (!rootShadowHost || !this.doc.contains(rootShadowHost));
       if (!n.parentNode || notInDoc) {
         return;
       }