Regression from "Fix cross joins in member_owners_excluding_project_bots"

The following discussion from !133391 (merged) should be addressed:

• @j.castillo started a discussion:

  @tkuah We received a support ticket regarding this line, this is the error we see:

  undefined method `project_bot?' for nil:NilClass

  https://new-sentry.gitlab.net/organizations/gitlab/issues/393901/

152 errors