Fix null issueLinks for issue notes

What does this MR do and why?

The MR adds a guard for null nodes. This will fix the problem:

🐛Problem (console)	🐛Problem (GraphQL)

Screenshots or screen recordings

✅ The modal will load properly even for null issueLinks

How to set up and validate locally

Setup

1. Have the related FF enabled: echo "Feature.enable(:pipeline_security_dashboard_graphql)" | rails c
2. You'll need an EE License
3. You'll need to have runners enabled (See $2408961 for setting up a runner)
4. Import https://gitlab.com/gitlab-examples/security/security-reports
5. Run a pipeline on master

Validation

1. Go to the pipeline's security report tab
2. Click on a finding with null issueLinks
3. Verify that the modal renders as expected

Numbered steps to set up and validate the change are strongly suggested.

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 #390572 (closed)