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

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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]

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