### 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 testProjectOverride() {
        EsqlQueryResponse results = run("from test | eval cnt = count | rename count as data | keep cnt, data");
        logger.info(results);
        Assert.assertEquals(40, getValuesList(results).size());
        assertThat(results.columns(), contains(new ColumnInfo("cnt", "long"), new ColumnInfo("data", "long")));
        for (List<Object> values : getValuesList(results)) {
            assertThat(values.get(1), is(values.get(0)));
        }
    }
    public void testRefreshSearchIdleShards() throws Exception {
        String indexName = "test_refresh";
        int numShards = between(1, 2);
        assertAcked(
            client().admin()
                .indices()
                .prepareCreate(indexName)
                .setSettings(
                    Settings.builder()
                        .put(IndexSettings.INDEX_SEARCH_IDLE_AFTER.getKey(), 0)
                        .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, numShards)
                        .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
                        .put("index.routing.rebalance.enable", "none")
                )
                .get()
        );
        ensureYellow(indexName);
        AtomicLong totalValues = new AtomicLong();
        CountDownLatch latch = new CountDownLatch(1);
        AtomicBoolean stopped = new AtomicBoolean();
        Thread indexingThread = new Thread(() -> {