DocPreviewCleanup should not error when no preview is found

[ranocha]

As discussed on Slack.

[sloede]

Thanks a lot for doing this! If it works, should we also inform the people at Documenter about this fix?

[ranocha]

If it works, should we also inform the people at Documenter about this fix?

Sounds reasonable to me (after testing this change). The relevant section in the docs of Documenter.jl is https://juliadocs.github.io/Documenter.jl/stable/man/hosting/#gh-pages-Branch (source at https://github.com/JuliaDocs/Documenter.jl/blob/master/docs/src/man/hosting.md#gh-pages-branch). We might also consider amking a PR to some clima projects where similar scripts are used (e.g. https://github.com/CliMA/TimeMachine.jl/blob/master/.github/workflows/DocCleanup.yml)

[codecov]

Codecov Report

Merging #488 (840a4f2) into main (d3421da) will not change coverage.
The diff coverage is n/a.

❗ Current head 840a4f2 differs from pull request most recent head a7445b5. Consider uploading reports for the commit a7445b5 to get more accurate results

@@           Coverage Diff           @@
##             main     #488   +/-   ##
=======================================
  Coverage   94.74%   94.74%           
=======================================
  Files         103      103           
  Lines       11439    11439           
=======================================
  Hits        10838    10838           
  Misses        601      601           

Flag	Coverage Δ
unittests	94.74% <ø> (ø)

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