### 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/compute/src/main/java/org/elasticsearch/compute/operator/OrdinalsGroupingOperator.java
private BytesRef currentTerm;
        private long currentOrd = -1;
        private final List<GroupingAggregator> aggregators;
        private final BitArray ords;
        private final SortedSetDocValues dv;
        AggregatedResultIterator(List<GroupingAggregator> aggregators, BitArray ords, SortedSetDocValues dv) {
            this.aggregators = aggregators;
            this.ords = ords;
            this.dv = dv;
        }
        int currentPosition() {
            assert currentOrd != Long.MAX_VALUE : "Must not read position when iterator is exhausted";
            return Math.toIntExact(currentOrd);
        }
        boolean next() throws IOException {
            currentOrd = ords.nextSetBit(currentOrd + 1);
            if (currentOrd < Long.MAX_VALUE) {
                currentTerm = dv.lookupOrd(currentOrd);
                return true;
            } else {
                currentTerm = null;
                return false;
            }
        }
    }
    private static class ValuesAggregator implements Releasable {