### 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 testProjectConstant() {
        EsqlQueryResponse results = run("from test | eval x = 1 | keep x");
        assertThat(results.columns(), equalTo(List.of(new ColumnInfo("x", "integer"))));
        assertThat(getValuesList(results).size(), equalTo(40));
        assertThat(getValuesList(results).get(0).get(0), equalTo(1));
    }
    public void testStatsOverConstant() {
        EsqlQueryResponse results = run("from test | eval x = 1 | stats x = count(x)");
        assertThat(results.columns(), equalTo(List.of(new ColumnInfo("x", "long"))));
        assertThat(getValuesList(results).size(), equalTo(1));
        assertThat(getValuesList(results).get(0).get(0), equalTo(40L));
    }
    public void testRow() {
        long value = randomLongBetween(0, Long.MAX_VALUE);
        EsqlQueryResponse response = run("row " + value);
        assertEquals(List.of(List.of(value)), getValuesList(response));
    }
    public void testFromStatsGroupingAvgWithSort() {
        testFromStatsGroupingAvgImpl("from test | stats avg(count) by data | sort data | limit 2", "data", "avg(count)");
    }
    public void testFromStatsGroupingAvg() {
        testFromStatsGroupingAvgImpl("from test | stats avg(count) by data", "data", "avg(count)");
    }
    public void testFromStatsGroupingAvgWithAliases() {
        testFromStatsGroupingAvgImpl("from test | eval g = data | stats f = avg(count) by g", "g", "f");
    }
    private void testFromStatsGroupingAvgImpl(String command, String expectedGroupName, String expectedFieldName) {
        try (EsqlQueryResponse results = run(command)) {
            logger.info(results);