avoid error on bad book data in discover page

fedireads/views.py

@@ -143,7 +143,7 @@ def books_page(request):
     if request.user.is_authenticated:
         recent_books = models.Edition.objects.filter(
             ~Q(shelfbook__shelf__user=request.user),
-            id__in=[b.id for b in recent_books],
+            id__in=[b.id for b in recent_books if b],
         )
 
     data = {