Fix MembersFinder to never include group invitees.
Fix MembersFinder to never include group invitees.

This was the default behavior, because all members with `user_id` set to
`null` were excluded.

jirutka/gitlabhq,dreampet/gitlab,dreampet/gitlab,mmkassem/gitlabhq,axilleas/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,stoplightio/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,dreampet/gitlab,jirutka/gitlabhq,iiet/iiet-git,dreampet/gitlab