gh-1850 fix nil-pointer in segment cursor

[etiennedi]

There was a small data race where a compaction could have already
occurred/finished between creating the array and initializing it, thus
leaving empty slots.

There is no test yet, as this will be tested through the stress-test
pipelines together with a potential fix for #1849

fixes #1850

Test for this will be added to the stress-testing pipeline.

[codecov]

Codecov Report

Merging #1859 (77156e6) into master (01a8262) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1859      +/-   ##
==========================================
+ Coverage   66.06%   66.12%   +0.05%     
==========================================
  Files         411      411              
  Lines       30915    30931      +16     
==========================================
+ Hits        20425    20454      +29     
+ Misses       8691     8678      -13     
  Partials     1799     1799              

Flag	Coverage Δ
integration	67.42% <100.00%> (+0.14%)	⬆️
unittests	66.12% <100.00%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
adapters/repos/db/lsmkv/cursor_segment_replace.go	85.96% <100.00%> (ø)
adapters/repos/db/vector/hnsw/insert.go	74.02% <0.00%> (-1.65%)	⬇️
adapters/repos/db/lsmkv/memtable.go	75.00% <0.00%> (-0.17%)	⬇️
adapters/repos/db/lsmkv/binary_search_tree.go	100.00% <0.00%> (ø)
...dapters/repos/db/lsmkv/segment_group_compaction.go	67.71% <0.00%> (+0.78%)	⬆️
usecases/sharding/state.go	66.99% <0.00%> (+0.97%)	⬆️
adapters/repos/db/shard_write_batch_objects.go	77.27% <0.00%> (+1.51%)	⬆️
adapters/repos/db/lsmkv/bucket.go	64.98% <0.00%> (+2.29%)	⬆️
adapters/repos/db/lsmkv/binary_search_tree_map.go	100.00% <0.00%> (+2.70%)	⬆️
adapters/repos/db/lsmkv/bucket_options.go	70.96% <0.00%> (+4.30%)	⬆️
... and 1 more

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