bug: last page would result in error

Signed-off-by: Nick Gerakines <[e-mail]>

src/cache.rs

···
         let feed_chunks = inner.cached_feeds.get(feed_id)?;
 
         if page > feed_chunks.len() {
-            return None;
+            return Some(vec![]);
         }
 
         feed_chunks.get(page).cloned()