CNDB-11713: index nulls in sai to optimize hybrid ANN

[michaeljmarshall]

This is a WIP. I am using it to run tests at the moment.

What is the issue

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What does this PR fix and why was it fixed

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Checklist before you submit for review

• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits

[sonarqubecloud]

Quality Gate passed

Issues
13 New issues
0 Accepted issues

Measures
0 Security Hotspots
88.1% Coverage on New Code
8.1% Duplication on New Code

See analysis details on SonarQube Cloud

[cassci-bot]

❌ Build ds-cassandra-pr-gate/PR-1455 rejected by Butler

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34 new test failure(s) in 2 builds
See build details here

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Found 34 new test failures

Showing only first 15 new test failures

Test	Explanation	Branch history	Upstream history
...dra.distributed.test.ReadRepairSliceQueriesTest	regression	🔴🔵	🔵🔵🔵🔵🔵🔵🔵
...ToolEnableDisableBinaryTest.testMaybeChangeDocs	regression	🔴🔵	🔵🔵🔵🔵🔵🔵🔵
...list,wide=false,scenario=MIXED_QUERY]	regression	🔴🔵	🔵🔵🔵🔵🔵🔵🔵
...map<date,date>,wide=false,scenario=MIXED_QUERY]	regression	🔴🔵	🔵🔵🔵🔵🔵🔵🔵
...reTest.testFailedTermsReaderOnMultiIndexesQuery	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...herTest.testEqQueriesAgainstStringIndex {p0=aa}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...herTest.testEqQueriesAgainstStringIndex {p0=ba}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...herTest.testEqQueriesAgainstStringIndex {p0=ca}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...herTest.testEqQueriesAgainstStringIndex {p0=db}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...herTest.testEqQueriesAgainstStringIndex {p0=eb}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...dexSearcherTest.testUnsupportedOperator {p0=aa}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...dexSearcherTest.testUnsupportedOperator {p0=ba}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...dexSearcherTest.testUnsupportedOperator {p0=ca}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...dexSearcherTest.testUnsupportedOperator {p0=db}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵
...dexSearcherTest.testUnsupportedOperator {p0=dc}	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵

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Found 695 known test failures