### 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
BigArrays bigArrays,
            DriverContext driverContext
        ) {
            this.extractor = new ValuesSourceReaderOperator(sources, docChannel, groupingField);
            this.aggregator = new HashAggregationOperator(
                aggregatorFactories,
                () -> BlockHash.build(List.of(new GroupSpec(channelIndex, sources.get(0).elementType())), bigArrays, maxPageSize, false),
                driverContext
            );
        }
        void addInput(Page page) {
            extractor.addInput(page);
            Page out = extractor.getOutput();
            if (out != null) {
                aggregator.addInput(out);
            }
        }
        void finish() {
            aggregator.finish();
        }
        Page getOutput() {
            return aggregator.getOutput();
        }
        @Override
        public void close() {
            Releasables.close(extractor, aggregator);
        }
    }
}