### 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
}
    public void testFilterWithNullAndEvalFromIndex() {
        
        client().prepareBulk().add(new IndexRequest("test").id("no_count").source("data", 12, "data_d", 2d, "color", "red")).get();
        client().admin().indices().prepareRefresh("test").get();
        
        EsqlQueryResponse results = run("from test");
        Assert.assertEquals(41, getValuesList(results).size());
        results = run("from test | eval newCount = count + 1 | where newCount > 1");
        logger.info(results);
        Assert.assertEquals(40, getValuesList(results).size());
        assertThat(results.columns(), hasItem(equalTo(new ColumnInfo("count", "long"))));
        assertThat(results.columns(), hasItem(equalTo(new ColumnInfo("count_d", "double"))));
        assertThat(results.columns(), hasItem(equalTo(new ColumnInfo("data", "long"))));
        assertThat(results.columns(), hasItem(equalTo(new ColumnInfo("data_d", "double"))));
        assertThat(results.columns(), hasItem(equalTo(new ColumnInfo("time", "long"))));
        
        client().prepareBulk().add(new DeleteRequest("test").id("no_count")).setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE).get();
        results = run("from test");
        Assert.assertEquals(40, getValuesList(results).size());
    }
    public void testMultiConditionalWhere() {
        EsqlQueryResponse results = run(
            "from test | eval abc = 1+2 | where (abc + count >= 44 or data_d == 2) and data == 1 | keep color, abc"
        );
        logger.info(results);
        Assert.assertEquals(10, getValuesList(results).size());
        Assert.assertEquals(2, results.columns().size());
        for (List<Object> values : getValuesList(results)) {
            assertThat((String) values.get(0), equalTo("green"));