fix: issue with k > length in various backends

[stephantul]

This PR fixes an issue with extremely low item count and k. If k was higher than the number of items in the database, some backends would crash

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
tests/test_vicinity.py	100.00% <100.00%> (ø)
vicinity/backends/faiss.py	89.83% <100.00%> (ø)
vicinity/backends/hnsw.py	95.45% <100.00%> (-2.83%)	⬇️
vicinity/backends/usearch.py	96.96% <100.00%> (ø)
vicinity/backends/voyager.py	96.82% <100.00%> (ø)

... and 1 file with indirect coverage changes

[Pringled]

LGTM