[omemo] Return None on empty result

omemo/omemo/litesessionstore.py

@@ -53,8 +53,7 @@ class LiteSessionStore(SessionStore):
         c = self.dbConn.cursor()
         c.execute(q, (device_id, ))
         result = c.fetchone()
-
-        return result[0]
+        return result[0] if result else None
 
     def getActiveDeviceTuples(self):
         q = "SELECT recipient_id, device_id FROM sessions WHERE active = 1"