bug fix for non-normalized dot product vectors returning no matches and causing assertion to fail if using hierarchy

[MarkWolters]

This PR resolves issue #713.

When dot product vsf is used for graph search in conjunction with enabling hierarchy this can result in a failed Assertion when no candidate is found to be sufficiently close to the search vector at the level being searched, in this case where level > 0.

Any candidate whose score is < 0 is silently dropped, never added to approximateResults. If every candidate reachable from the entry point within that hierarchy layer happens to score negative for a given query, approximateResults stays empty and the size() == 1 assert fires.

0.0f is only a valid "accept everything" sentinel for similarity functions that are mathematically bounded to (0, 1] — COSINE and EUCLIDEAN are. DOT_PRODUCT is not bounded unless callers pre-normalize vectors to unit length. VectorDotProductWithLengthTest.testTrueDotproduct (the failing Cassandra unit test that surfaced this issue) deliberately uses non-unit 2D vectors with components in [-100, 100].

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