fix(paginator): guard getVisibleRange against not-yet-loaded documents

Second member of the 50f56ee guard family: the scroll-anchor tick
(#afterScroll) can run before the iframe document is parsed, throwing
'createTreeWalker' on null body. getVisibleRange now returns null for
that tick and #afterScroll preserves the previous visibility state.
Observed live in the Android app on PalmDOC open.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "foliate-js",
-  "version": "0.5.1",
+  "version": "0.5.2",
   "description": "Render e-books in the browser",
   "repository": {
     "type": "git",