### 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
assertThat(actualDocs, equalTo(allDocs.stream().limit(limit).toList()));
        }
    }
    public void testEvalWithNullAndAvg() {
        EsqlQueryResponse results = run("from test | eval nullsum = count_d + null | stats avg(nullsum)");
        logger.info(results);
        Assert.assertEquals(1, results.columns().size());
        Assert.assertEquals(1, getValuesList(results).size());
        assertEquals("avg(nullsum)", results.columns().get(0).name());
        assertEquals("double", results.columns().get(0).type());
        assertEquals(1, getValuesList(results).get(0).size());
        assertNull(getValuesList(results).get(0).get(0));
    }
    public void testFromStatsLimit() {
        EsqlQueryResponse results = run("from test | stats ac = avg(count) by data | limit 1");
        logger.info(results);
        assertThat(results.columns(), contains(new ColumnInfo("ac", "double"), new ColumnInfo("data", "long")));
        assertThat(getValuesList(results), contains(anyOf(contains(42.0, 1L), contains(44.0, 2L))));
    }
    public void testFromLimit() {
        try (EsqlQueryResponse results = run("from test | keep data | limit 2")) {
            logger.info(results);
            assertThat(results.columns(), contains(new ColumnInfo("data", "long")));
            assertThat(getValuesList(results), contains(anyOf(contains(1L), contains(2L)), anyOf(contains(1L), contains(2L))));
        }
    }
    public void testDropAllColumns() {
        EsqlQueryResponse results = run("from test | keep data | drop data | eval a = 1");
        logger.info(results);
        assertThat(results.columns(), hasSize(1));