Fix closing of external issues

What does this MR do and why?

Allow closing of external issues even when GitLab issues are disabled for the project.

How to set up and validate locally

1. Enable Jira integration and disable the internal issue tracker. (You don't need an actual Jira account, just setup any URL and it would still generate references that match the pattern ABC-123)
2. Create an MR with a description that closes a Jira issue. (e.g. Closes JIRA-123)
3. Check the MR widget and confirm that it says "Closes issue JIRA-123"

Before	After

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.

Related to #376161 (closed)