Wrong data type of returned `running_sum` when sum_hit_scores == 0

[chienhsiang]

Hi Alex,

Should the returned running_sum be an array instead of zero in this line? https://github.com/MaayanLab/blitzgsea/blob/54ca73de4c89be3824562f2ef8998aa0f179fbf7/blitzgsea/init.py#L49

I ran into error when sum_hit_scores == 0. It turns out that this line assumes that runningsum is an array: https://github.com/MaayanLab/blitzgsea/blob/54ca73de4c89be3824562f2ef8998aa0f179fbf7/blitzgsea/init.py#L78