### Description

Not sure if it's a regression or not but when grouping by just one keyword field, the null group is not returned:
```
from employees | stats by gender | sort gender 
```
should return [F, M, null] yet only [F, M] are.

It looks like the hashing function (whether ordinal or not) ends up dropping null along the way. 
This doesn't happen with multiple keys as long as the keyword is not the first key:

```
from employees | stats by languages, gender | sort desc
```

```
null | null
null | F
null | M
etc...
```

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/EsqlActionIT.java
Assert.assertEquals(1, getValuesList(results).size());
        assertEquals("count(*)", results.columns().get(0).name());
        assertEquals("long", results.columns().get(0).type());
        assertEquals("data", results.columns().get(1).name());
        assertEquals("long", results.columns().get(1).type());
        var values = getValuesList(results).get(0);
        assertEquals(2, values.size());
        assertEquals(20, (long) values.get(0));
        assertEquals(2L, (long) values.get(1));
    }
    public void testFromStatsEvalWithPragma() {
        assumeTrue("pragmas only enabled on snapshot builds", Build.current().isSnapshot());
        EsqlQueryResponse results = run("from test | stats avg_count = avg(count) | eval x = avg_count + 7");
        logger.info(results);
        Assert.assertEquals(1, getValuesList(results).size());
        assertEquals(2, getValuesList(results).get(0).size());
        assertEquals(50, (double) getValuesList(results).get(0).get(results.columns().indexOf(new ColumnInfo("x", "double"))), 1d);
        assertEquals(43, (double) getValuesList(results).get(0).get(results.columns().indexOf(new ColumnInfo("avg_count", "double"))), 1d);
    }
    public void testWhere() {
        EsqlQueryResponse results = run("from test | where count > 40");
        logger.info(results);
        Assert.assertEquals(30, getValuesList(results).size());
        var countIndex = results.columns().indexOf(new ColumnInfo("count", "long"));
        for (List<Object> values : getValuesList(results)) {
            assertThat((Long) values.get(countIndex), greaterThan(40L));
        }
    }
    public void testProjectWhere() {
        EsqlQueryResponse results = run("from test | keep count | where count > 40");