Fix users cannot access job detail page when deployable does not exist

What does this MR do?

As some on-prem customers and sentry reports, sometimes deployment.deployable becomes nil. Because of that, JobController#show raises an exception and it disturbs user's workflow.

This MR gracefully handles the case by explicitly checking the object existence.

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/65216

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
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Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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