if there are no cards to shift, don't try

anki/sched.py

@@ -809,6 +809,7 @@ where id = :id""", vals)
                 "select min(due) from cards where due >= ? and type = 0 "
                 "and id not in %s" % scids,
                 start)
+            if low is not None:
                 shiftby = high - low + 1
                 self.deck.db.execute("""
 update cards set mod=?, due=due+? where id not in %s