Handling of SPARQL `UNDEF` in JSON responses

[lu-pl]

The SPARQL 1.2 Query Results JSON Format states in 3.2.1 "bindings":

The value of the "bindings" member is an array with zero or more elements, one element per query solution. Each query solution is a JSON object. 3.2.1 "select bindings"

Given a query like

select ?x where {values ?x {1 2 UNDEF}}

a Qlever SPARQL endpoint like e.g. https://qlever.dev/api/wikidata returns the following JSON content:

{
    "head": {
        "vars": [
            "x"
        ]
    },
    "results": {
        "bindings": [
            {
                "x": {
                    "datatype": "http://www.w3.org/2001/XMLSchema#int",
                    "type": "literal",
                    "value": "1"
                }
            },
            {
                "x": {
                    "datatype": "http://www.w3.org/2001/XMLSchema#int",
                    "type": "literal",
                    "value": "2"
                }
            },
            null
        ]
    },
    "meta": {
        "query-time-ms": 5,
        "result-size-total": 3
    }
}

I think that returning null for SPARQL UNDEF instead of {} can be considered a spec violation.