fix: fix prefetch conversion, fix local mode query batch points offse…

[joein]

Two things were broken:

1. In local mode in query_batch_points during iteration over requests we were passing to local_collection.query_points method offset which could be None, however, it does not correspond to the interface of local_collection.query_points which accepts only int offset
2. QueryRequest conversion was broken because it was not not properly tested. We missed a case when QueryRequest.prefetch is a single object and not a list, grpc.QueryPoints, in its turn, accepts only list of prefetches.

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