Fix reviewer roulette when no-one is in a category
Before:
spin_for_person([], random: nil)
#=> 0
After:
spin_for_person([], random: nil)
#=> nil
Prevents https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27889#note_165519665.
Before:
spin_for_person([], random: nil)
#=> 0
After:
spin_for_person([], random: nil)
#=> nil
Prevents https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27889#note_165519665.