Hi Haddock: don't try to copy artifacts that don't exist

[sheaf]

This fixes a bug in which, if we run cabal haddock without having run cabal build first, then we could try to copy over artifacts that don't exist, which would cause a build failure. Simply not attempting to copy non-existing artifacts fixes the issue.

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Template Α: This PR modifies behaviour or interface

Include the following checklist in your PR:

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[ulysses4ever]

This fixes a bug...

A general guideline, which is also included in our CONTRIBUTING.md, is to have a ticket before a PR. It'd be great if you could find a corresponding issue (if one exists) and link it. Otherwise, opening one with a reproducer is also a good option.

[sheaf]

This fixes a bug...

A general guideline, which is also included in our CONTRIBUTING.md, is to have a ticket before a PR. It'd be great if you could find a corresponding issue (if one exists) and link it. Otherwise, opening one with a reproducer is also a good option.

I have opened #11001 to track the issue. I added a test and a changelog entry as well.

[mergify]

This pull request has been removed from the queue for the following reason: pull request branch update failed.

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You should update or rebase your pull request manually. If you do, this pull request will automatically be requeued once the queue conditions match again.
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[mergify]

This pull request has been removed from the queue for the following reason: pull request branch update failed.

The pull request can't be updated.

You should update or rebase your pull request manually. If you do, this pull request will automatically be requeued once the queue conditions match again.
If you think this was a flaky issue, you can requeue the pull request, without updating it, by posting a @mergifyio requeue comment.