gangway: add nil check on refs

[Prucek]

Periodics do not have refs, this caused panics.

[k8s-ci-robot]

Hi @Prucek. Thanks for your PR.

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[petr-muller]

/ok-to-test

[petr-muller]

/retest

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: petr-muller, Prucek, stevekuznetsov

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/prow/blob/main/OWNERS)~~ [petr-muller,stevekuznetsov] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment