fix twitter when all refreshed posts are gone

tasks.py

@@ -186,6 +186,7 @@ def delete_from_account(account_id):
     if account.service == 'twitter':
         action = lib.twitter.delete
         posts = refresh_posts(posts)
+        if posts:
             eligible = random.choice(list(  # nosec
                 (post for post in posts if
                 (not account.policy_keep_favourites or not post.favourite)