Fix issue list for closed issues without closed_at

What does this MR do and why?

Prevents a 500 in the dashboard issue list and fixes wrong timestamp being used in vue issue list when a closed issue does not have a closed_at value.

This can happen when an issue is imported from GitHub or for old issues that were closed before we added the closed_at column.

This fixes a regression from !90570 (merged)

How to set up and validate locally

1. Simulate missing closed_at by editing a closed issue from the Rails console.
2. View the issue in the dashboard issues list and confirm that the page does not 500
3. View the issue in the project issue list and confirm that the timestamp is correct. Before this MR, it just used current time.

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 #366580 (closed)