Fix `Group#owned_by?` and `#has_user?` methods to handle nil

What does this MR do and why?

Addressing: https://gitlab.com/gitlab-com/sec-sub-department/section-sec-request-for-help/-/issues/179 (internal)

Closes #437123 (closed)

Fix Group#owned_by? and Group/ProjectTeam#has_user? methods to handle nil

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