If ARTIFACTS_PATH returns no files, all files are uploaded

[birdsarah]

A few weeks after setting up artifacts with export ARTIFACTS_PATHS="$(git ls-files -o | tr "\n" ":")" doing the work of uploading the assets created during the test run, I decided to tweak my .gitignore.

This had the impact of meaning that no files were returned by git ls-files -o. Instead of no files being uploaded, all files in my test directory were uploaded.

This may be intentional, but I think it should, at least, be documented.