IS NULL filter operator

[jamesfisher-geo]

Describe the bug
IS NULL is included in the filter extension logic. However, I cannot get it to work with cql2-json or cql2-text.

stac-fastapi-elasticsearch-opensearch/stac_fastapi/core/stac_fastapi/core/extensions/filter.py

Line 74 in 2d6cb4d

IS_NULL = "isNull"

To Reproduce
Steps to reproduce the behavior:

1. docker compose up app-opensearch or docker compose up app-elasticsearch
2. python3 data_loader.py --base-url http://localhost:8082
3. Try the following:
   POST http://localhost:8082/search
   body

{
  "filter-lang": "cql2-json",
  "filter": {
            "op": "isNull",
            "args": [ { "property": "sentinel:data_coverage" } ]
          }

}

{
    "detail": "Error with cql2_json filter: Q() can only accept dict with a single query ({\"match\": {...}}). Instead it got ({})"
}

GET http://localhost:8082/search?filter=sentinel:data_coverage > 50 OR landsat:coverage_percent < 10 OR (sentinel:data_coverage IS NULL AND landsat:coverage_percent IS NULL)

{
    "detail": "Error with cql2_json filter: Q() can only accept dict with a single query ({\"match\": {...}}). Instead it got ({})"
}

Expected behavior
valid ItemCollection