Fix reviewer roulette when no-one is in a category
Fix reviewer roulette when no-one is in a category

This would return `people.size` before, when it should return `nil`.

stoplightio/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,iiet/iiet-git,mmkassem/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git