fix: dbsf zero variance

[pavelm10]

solves #871

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[generall]

python implementation of scoring should follow the original one in qdrant core, and there we do this:

https://github.com/qdrant/qdrant/blob/a0ea3caccff8dd24eab59692233f26974d51d426/lib/segment/src/common/score_fusion.rs#L99

Let's follow the reference implementation in this case

[pavelm10]

@generall thanks for review, changed the implementation.

[pavelm10]

@generall hi is there anything else to do?

[joein]

Hey @pavelm10
Thanks for the contribution, I think it is pretty much done now