Fix atom feed with push events for multiple tags

What does this MR do?

When the push event is for multiple tags, we cannot generate a link because we don't store the tags / refs.

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2599745

You can see an example of this type of event in https://gitlab.com/api/v4/projects/11903504/events:

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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Availability and Testing

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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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