when actual list zero lists, the apk should alwasy return 1.0;

[apmfifty]

1. when actual list zero lists, the apk should alwasy return 1.0; The current versio of code will always return 0.0 which is different from the code of function using pip install.
2. Besides, put the code at first to skip unncessary operation.

[codelucas]

I thought the right way to do it was if the actual list was length 1 we should not include the ndcg because that value will always be 1. The result is, if you have a lot of 1 sized lists your metric will be skewed.