### 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
}
        EsqlQueryResponse results = run("from test_index_patterns* | stats count(data), sum(count)");
        assertEquals(1, getValuesList(results).size());
        assertEquals(15L, getValuesList(results).get(0).get(0));
        assertEquals(120000L, getValuesList(results).get(0).get(1));
        results = run("from test_index_patterns_1,test_index_patterns_2 | stats count(data), sum(count)");
        assertEquals(1, getValuesList(results).size());
        assertEquals(10L, getValuesList(results).get(0).get(0));
        assertEquals(55000L, getValuesList(results).get(0).get(1));
        results = run("from test_index_patterns_1*,test_index_patterns_2* | stats count(data), sum(count)");
        assertEquals(1, getValuesList(results).size());
        assertEquals(10L, getValuesList(results).get(0).get(0));
        assertEquals(55000L, getValuesList(results).get(0).get(1));
        results = run("from test_index_patterns_*,-test_index_patterns_1 | stats count(data), sum(count)");
        assertEquals(1, getValuesList(results).size());
        assertEquals(10L, getValuesList(results).get(0).get(0));
        assertEquals(105000L, getValuesList(results).get(0).get(1));
        results = run("from * | stats count(data), sum(count)");
        assertEquals(1, getValuesList(results).size());
        assertEquals(55L, getValuesList(results).get(0).get(0));
        assertEquals(121720L, getValuesList(results).get(0).get(1));
        results = run("from test_index_patterns_2 | stats count(data), sum(count)");
        assertEquals(1, getValuesList(results).size());
        assertEquals(5L, getValuesList(results).get(0).get(0));
        assertEquals(40000L, getValuesList(results).get(0).get(1));
    }
    public void testOverlappingIndexPatterns() throws Exception {
        String[] indexNames = { "test_overlapping_index_patterns_1", "test_overlapping_index_patterns_2" };
        assertAcked(
            client().admin()