### 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
try (EsqlQueryResponse results = run("from test | stats avg(count) by data | sort data")) {
            logger.info(results);
            assertThat(results.columns(), hasSize(2));
            assertEquals("avg(count)", results.columns().get(0).name());
            assertEquals("double", results.columns().get(0).type());
            assertEquals("data", results.columns().get(1).name());
            assertEquals("long", results.columns().get(1).type());
            record Group(Long data, Double avg) {}
            List<Group> expectedGroups = List.of(new Group(1L, 42.0), new Group(2L, 44.0), new Group(99L, null), new Group(null, 12.0));
            List<Group> actualGroups = getValuesList(results).stream().map(l -> new Group((Long) l.get(1), (Double) l.get(0))).toList();
            assertThat(actualGroups, equalTo(expectedGroups));
        }
    }
    public void testFromStatsGroupingByKeyword() {
        EsqlQueryResponse results = run("from test | stats avg(count) by color");
        logger.info(results);
        Assert.assertEquals(2, results.columns().size());
        Assert.assertEquals(3, getValuesList(results).size());
        
        assertEquals("avg(count)", results.columns().get(0).name());
        assertEquals("double", results.columns().get(0).type());
        assertEquals("color", results.columns().get(1).name());
        assertEquals("keyword", results.columns().get(1).type());
        record Group(String color, double avg) {
        }
        List<Group> expectedGroups = List.of(new Group("blue", 42.0), new Group("green", 44.0), new Group("red", 43));
        List<Group> actualGroups = getValuesList(results).stream()
            .map(l -> new Group((String) l.get(1), (Double) l.get(0)))
            .sorted(comparing(c -> c.color))
            .toList();