973: underprediction/overprediction/etc. fails if all observations are above/below the medians

[sbfnk]

Description

This PR closes #973.

Checklist

• [X] My PR is based on a package issue and I have explicitly linked it.
• [X] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [X] I have tested my changes locally.
• [X] I have added or updated unit tests where necessary.
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• [X] I have built the package locally and run rebuilt docs using roxygen2.
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[sbfnk]

lint failure is unrelated and due to https://github.com/RcppCore/Rcpp/issues/1341

[nikosbosse]

Left one comment suggesting to add your original example as a test. Was the original issue actually about values being above/below the median? Or was it ultimately something else?