* create initial strucutres
* clippy
* start field-query refactoring
* start field-query refactoring (2/N)
* start field-query refactoring (3/N): duplicate is_empty/null condiftions as field condition
* start field-query refactoring (4/N): re-instate is_empty fallback in case new index is not built yet
* filter for is_empty/is_null
* implement add/remove point
* upd schema
* open and create of null-index
* create null-index
* fix test
* Update lib/segment/src/index/query_optimization/condition_converter.rs
Co-authored-by: Tim Visée <tim+github@visee.me>
* unit test for null-index
* more unit tests
* add openapi tests
* fmt
* fix for integartion tests
* rabbit review fix
* make [null] non-empty
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Co-authored-by: Tim Visée <tim+github@visee.me>
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