Address review: match the sibling adapter guard in the lnw skip (#91)

The manga script's refresh guards both the adapter and its method; the
novel guard skipped the adapter null-check. Provably non-null today, but
the sibling shape is the convention to match.

userscript/novel-bookmark.user.js

@@ -862,7 +862,7 @@
     // before the due filter records no freshness timestamp and consumes no
     // per-navigation batch slot.
     const adapter = adapterFor(site);
-    if (!adapter.latestChapterFromAnchors) return;
+    if (!adapter || !adapter.latestChapterFromAnchors) return;
 
     const checked = loadLastChecked();
     const now = Date.now();