Fix Amazon Q disconnect for already destroyed app

What does this MR do and why?

Backports this MR: Fix Amazon Q disconnect for already destroyed app (!187949 - merged)

The problem is:

• An application is destroyed
• Which means Doorkeeper::Application.find_by_id(ai_settings.amazon_q_oauth_application_id) is nil
• So it means that destroy_oauth_application! returns nil and execute returns early with an empty error
• But there's no validation error
• This is why we see a failure with an unknown error

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).