fix: make empty listings actually empty

Some empty listings were misbehaving and claiming that they had a single
entry that was `undefined`. This was a lie, and they have been told off.

src/listings/listing.ts

@@ -230,7 +230,8 @@ export default class Listing<T> {
     if (page.next) {
       return page.next;
     } else if (page.fetcher) {
-      return await page.fetcher.fetch(ctx);
+      const next = await page.fetcher.fetch(ctx);
+      return next.arr.length > 0 ? next : null;
     } else {
       return null;
     }