Fix 500 error for X509 commits without a certificate

What does this MR do and why?

Contributes to #421683 (closed)

Problem

x509_certificate can be undefined when the certificate is invalid. That leads to undefined method exceptions.

Solution

Only display certificate extra details when the certificate is present.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Visit http://127.0.0.1:3000/gitlab-org/gitlab-test/-/tags/ in GDK
2. You should see a list of tags and a signature badge (without the fix, the page returns a 500 error)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.