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. https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/blob/2d6cb4dabb5847fa22e4734187779dcbcb7b41b9/stac_fastapi/core/stac_fastapi/core/extensions/filter.py#L74

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" } ]
         }

}

```json
{
    "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

[jamesfisher-geo]

For full implementation we are waiting on a new version of pygeofilter

[jamesfisher-geo]

It is possible that we could migrate the cql2 parsing logic to use cql-rs Python bindings