Handle build.runner being nil when constructing JWT

What does this MR do and why?

Fixes a regression introduced in !116867 (merged). Changelog is not added because the regression was introduced in the same release.

Related to #409621 (closed)

Screenshots or screen recordings

Pending job:

Before fix (signed out):

After fix (signed out):

How to set up and validate locally

1. Create a pending job (e.g. by adding a tag to the job so it doesn't get picked up)
2. Open the trace while logged out (add /trace.json to the path)
3. Check that the status for the request is 204 and not 500

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.