### 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 ValuesAggregator valuesAggregator;
    public OrdinalsGroupingOperator(
        List<ValueSourceInfo> sources,
        int docChannel,
        String groupingField,
        List<GroupingAggregator.Factory> aggregatorFactories,
        int maxPageSize,
        BigArrays bigArrays,
        DriverContext driverContext
    ) {
        Objects.requireNonNull(aggregatorFactories);
        boolean bytesValues = sources.get(0).source() instanceof ValuesSource.Bytes;
        for (int i = 1; i < sources.size(); i++) {
            if (sources.get(i).source() instanceof ValuesSource.Bytes != bytesValues) {
                throw new IllegalStateException("ValuesSources are mismatched");
            }
        }
        this.sources = sources;
        this.docChannel = docChannel;
        this.groupingField = groupingField;
        this.aggregatorFactories = aggregatorFactories;
        this.ordinalAggregators = new HashMap<>();
        this.maxPageSize = maxPageSize;
        this.bigArrays = bigArrays;
        this.driverContext = driverContext;
    }
    @Override
    public boolean needsInput() {
        return finished == false;
    }