ES|QL query produces column with empty name and null value

[luigidellaquila]

With test dataset

POST /_query
{
  "query":"from employees | stats  foo = min(emp_no), bar = count_distinct(emp_no) | rename bar AS bug_here, foo AS baz, baz AS bug_here | eval bug_here = \"something\""
}

{
    "took": 5,
    "is_partial": false,
    "columns": [
        {
            "name": "",                  <<<-----
            "type": "null"               <<<-----
        },
        {
            "name": "bug_here",
            "type": "keyword"
        }
    ],
    "values": [
        [
            null,
            "something"
        ]
    ]
}

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)