Throw error when pos is empty

[willard-yuan]

@filipradenovic Hi filipradenovic, I find there is a bug when pos is empty in evaluate.py#L80. If there is no positive result in the ranking result, such as rank@10, the pos is empty, then it will throw an error. How you fix the bug? I'm on the way to try to understand the MAP in the file of evaluate.py.

Thanks.