Occasional failures with test_g123_...[af1_sim]

[leehart]

https://github.com/malariagen/malariagen-data-python/actions/runs/12260589223/job/34205572177?pr=694

=========================== short test summary info ============================
FAILED tests/anoph/test_g123.py::test_g123_gwss_with_default_sites[af1_sim] - IndexError: index 0 is out of bounds for axis 0 with size 0
FAILED tests/anoph/test_g123.py::test_g123_gwss_with_phased_sites[af1_sim] - IndexError: index 0 is out of bounds for axis 0 with size 0
FAILED tests/anoph/test_g123.py::test_g123_calibration[af1_sim] - IndexError: index -1 is out of bounds for axis 0 with size 0
============ 3 failed, 521 passed, 2 warnings in 282.92s (0:04:42) =============