### 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
assertEquals(1, getValuesList(results).get(0).size());
        assertEquals(0.034d, (double) getValuesList(results).get(0).get(0), 0.001d);
    }
    public void testUngroupedCountAll() {
        EsqlQueryResponse results = run("from test | stats count(*)");
        logger.info(results);
        Assert.assertEquals(1, results.columns().size());
        Assert.assertEquals(1, getValuesList(results).size());
        assertEquals("count(*)", results.columns().get(0).name());
        assertEquals("long", results.columns().get(0).type());
        var values = getValuesList(results).get(0);
        assertEquals(1, values.size());
        assertEquals(40, (long) values.get(0));
    }
    public void testUngroupedCountAllWithFilter() {
        EsqlQueryResponse results = run("from test | where data > 1 | stats count(*)");
        logger.info(results);
        Assert.assertEquals(1, results.columns().size());
        Assert.assertEquals(1, getValuesList(results).size());
        assertEquals("count(*)", results.columns().get(0).name());
        assertEquals("long", results.columns().get(0).type());
        var values = getValuesList(results).get(0);
        assertEquals(1, values.size());
        assertEquals(20, (long) values.get(0));
    }
    @AwaitsFix(bugUrl = "tracking down a 64b(long) memory leak")
    public void testGroupedCountAllWithFilter() {
        EsqlQueryResponse results = run("from test | where data > 1 | stats count(*) by data | sort data");
        logger.info(results);
        Assert.assertEquals(2, results.columns().size());