[BUG] IS NULL returns wrong results for text fields

[Yury-Fridlyand]

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Import test data set (run from root of SQL plugin repo)

curl -s -XPOST localhost:9200/beer.stackexchange/_bulk -H 'Content-Type: application/x-ndjson' --data-binary @integ-test/src/test/resources/beer.stackexchange.json

2. Execute query like

select Id, Body from beer.stackexchange where Body is null LIMIT 5;

What is the expected behavior?
Result set should contain only lines where Body has null value

Actual result
Result set contains lines non-empty Body.

What is your host/environment?
Tested on 7cbb121cb3ad144a5e6d5b9acb9ae8e6da01688e (current main/HEAD).
OpenSearch 2.0