Fixed merge logic in hybrid query for multiple shards case

[martin-gaievski]

Description

Fix for scenario in hybrid query search when:

• concurrent segment search is enabled
• one of the shards does not have any results
• such shard appeared first in the sequence of multiple "merge shards results" calls

It's a gap in existing code, we only handle scenarios when merged result object is not empty and we're merging it with an empty result object.

The issue isn't trivial to repro because the sequence in which segment/shard results are passed to a merger isn't guaranteed, and we need to ingest documents and run search multiple times. Credit for providing a tool to replicate the issue goes to @eemmiirr, I used his script https://github.com/eemmiirr/opensearch-concurrent-segment-search-bug.

Related Issues

Resolves #875

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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