Split results in 0 selected tests, which leads to failed GitHub action

Hi,

We have integrated `pytest-split` as part of our CI pipeline and noticed the following behaviour.

We were running our tests in 7 splits, but after updating the `.test_durations` script (new tests being added), the 7th split did not select any test. This lead to a failed CI run since all the tests were deselected (see below).


Any idea why this could happen?

We tried to create more splits after this error was raised (increased from 7 to 10) and everything worked fine.

**We would like to understand why the 7th split did not select any test when `--splits 7` but everything worked well when `--splits 10`.**

Thanks in advance. Your package has become very useful in speeding up our CICD pipelines.

Start with the split-selection logic that reads `.test_durations` and the `--splits` option; reproduce the reported run with 7 splits and compare it with 10. Check the GitHub Actions output for the zero-selected-test case and add a regression test if the behavior is a bug. Done means the cause and expected handling are documented and tested.

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