fix(paginator): bail from expand() when the iframe document is torn down

A queued resize tick can run against a not-yet-parsed or unloaded
iframe; documentElement is null then and expand() threw inside the
ResizeObserver callback. Observed live in the Android app (twice in
row during boot and page turns). Follows the 50f56ee guard family.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "foliate-js",
-  "version": "0.5.0",
+  "version": "0.5.1",
   "description": "Render e-books in the browser",
   "repository": {
     "type": "git",