Fix missing stop entry for empty atom arrays

[padix-key]

If get_residue_start(add_exclusive_stop=True)/get_chain_starts(add_exclusive_stop=True) is called on an empty structure, the exclusive stop at [0] is missing. This PR fixes this

[codspeed-hq]

Merging this PR will not alter performance

✅ 98 untouched benchmarks

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_{Comparing padix-key:fix-segments (8900c8d) with main (fe75450)¹}

Footnotes

1. No successful run was found on main (c782ee5) during the generation of this report, so fe75450 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩