isNull operator in the /entities/search endpoint doesn't work with references

[gorbunkov]

Suppose we have a sample_City entity that has a reference to the sample_Country reference.

If we try to search cities where country is not set using the isNull operator:

POST http://localhost:8080/app/rest/v2/entities/sample_City/search

{
    "filter": {
      "conditions": [
                  {
                      "property": "country",
                      "operator": "isNull"
                  }
      ]
    },
    "returnCount": true
}

Then we won't get any result. For simple datatypes (string, integer, etc) the isNull operator seems to work.