Fix bug in batch iterators for the case where batch_size=0

[alonre24]

In brute force - instead of asserting that we hadn't computed the scores when the number of returned results by the batch iterator is 0, we should assert that the number of returned results by the iterator is 0 if the scores hadn't been computed yet (otherwise, we crash after asking for 0 results).
In HNSW - we should insert the entry point node to the candidates heap only when both the returned results count is 0 and the top_candidates_extra queue is empty. Otherwise, we might get that the entry point will be returned twice (if we ask for 0 results, and moved the entry point to the top_candidates_extra, and then reinsert the entry point to the candidates).

[codecov]

Codecov Report

Merging #147 (1356e4e) into main (8cbf10a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #147   +/-   ##
=======================================
  Coverage   92.60%   92.61%           
=======================================
  Files          39       39           
  Lines        1962     1963    +1     
=======================================
+ Hits         1817     1818    +1     
  Misses        145      145           

Impacted Files	Coverage Δ
...ecSim/algorithms/brute_force/bf_batch_iterator.cpp	100.00% <100.00%> (ø)
src/VecSim/algorithms/hnsw/hnsw_batch_iterator.cpp	100.00% <100.00%> (ø)

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[GuyAv46]

THANK YOU