[allisonlarson]

Description

The hook field in the lifecycle block is documented as being required, but when a task was being canonicalized, the block was being treated as empty if the hook wasn't set. This would prevent the validations from correctly returning an error to the user, and would instead ignore the lifecycle block completely and run the task without it. This change fixes corrects the canonicalization by correcting the Empty() check, and adds tests to ensure the struct is correctly canonicalized.

Testing & Reproduction steps

Added test cases to the unit tests, but reproduction is to define a task with an empty lifecycle block.

Links

Fixes #24837

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
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  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

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  backporting document.
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  in the majority of situations. The main exceptions are long-lived feature branches or merges where
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  within the public repository.

[Juanadelacuesta]

LGTM!

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