Adding a nil check for AlternateImageRepository

[bradmwilliams]

The ensureReleaseMirrorJob() method relies on release.Config.AlternateImageRepository being set. This PR performs a check to ensure that there is a value before creating a job that will never succeed:

Command:
  /bin/bash
  -c
      
        set -eu
        mkdir -p "${XDG_RUNTIME_DIR}"
        oc registry login
            
        oc image mirror --keep-manifest-list=$1 $2 $3
            
      
  true
  registry.ci.openshift.org/ocp/release:4.20.0-0.ci-2025-04-02-143700
  :4.20.0-0.ci-2025-04-02-143700
State:      Terminated
  Reason:   Error
  Message:  info: Using registry public hostname registry.ci.openshift.org
Saved credentials for registry.ci.openshift.org into /tmp/run/containers/auth.json
error: ":4.20.0-0.ci-2025-04-02-143700" is not a valid image reference: invalid reference format

  Exit Code:    1

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bradmwilliams, hoxhaeris

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [bradmwilliams,hoxhaeris]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[openshift-ci]

@bradmwilliams: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/govulncheck	227f54a	link	false	/test govulncheck
ci/prow/security	227f54a	link	false	/test security

Full PR test history. Your PR dashboard.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.