dont crash when there are no posts to refresh

lib/twitter.py

@@ -113,6 +113,9 @@ def fetch_acc(account, cursor, consumer_key=None, consumer_secret=None):
 
 
 def refresh_posts(posts):
+    if not posts:
+        return posts
+
     t = get_twitter_for_acc(posts[0].author)
     tweets = t.statuses.lookup(_id=",".join((post.twitter_id for post in posts)),
             trim_user = True, tweet_mode = 'extended')