chore: made code more robust to undefined expiry

[CommanderStorm]

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📝 Summary of changes done and why they are done

Not sure if this is just a bandait fix, but it gets the job done and is good under defensive programmingn anyhow.

• Resolves Domain name xxxxx will expire in NaN days #6618

[Copilot]

Pull request overview

This PR fixes a bug where domain expiry notifications could display "NaN days" when the expiry date is null or undefined. The fix adds a defensive check before calculating days remaining, preventing the notification from being sent with invalid data.

• Added validation to skip notifications when domain expiry is null/undefined
• Added regression tests for both null and undefined expiry scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
server/model/domain_expiry.js	Added check to return early if domain.expiry is falsy, preventing NaN calculation
test/backend-test/test-domain.js	Added two regression tests to verify notifications are not sent when expiry is null or undefined