Comparisons with empty strings

Hi, I am using Elasticsearch 6.6.0 and I noticed that this query:

    {
      "query": {
        "range": {
          "my_alphanumeric_field": {
            "from": "",
            "to": null,
            "include_lower": false,
            "include_upper": true,
            "boost": 1
          }
        }
      }
    }

returns nothing. Why? It looks strange, considering that Java statements like notEmptyString.compareTo("") return a positive number, so my expectation is that the result set contains all documents having not empty values for field "my_alphanumeric_field".
I would appreciate a clarification. Thanks.