### 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 testReturnNoNestedDocuments() throws IOException, ExecutionException, InterruptedException {
        var indexName1 = "test_nested_docs_1";
        var indexName2 = "test_nested_docs_2";
        var indices = List.of(indexName1, indexName2);
        var alias = "test-alias";
        int docsCount = randomIntBetween(50, 100);
        int[] countValuesGreaterThanFifty = new int[indices.size()];
        for (int i = 0; i < indices.size(); i++) {
            String indexName = indices.get(i);
            createNestedMappingIndex(indexName);
            countValuesGreaterThanFifty[i] = indexDocsIntoNestedMappingIndex(indexName, docsCount);
        }
        createAlias(indices, alias);
        var indexToTest = randomIntBetween(0, indices.size() - 1);
        var indexNameToTest = indices.get(indexToTest);
        
        assertNoNestedDocuments("from " + indexNameToTest, docsCount, 0L, 100L);
        
        assertNoNestedDocuments("from " + indexNameToTest + " | where data >= 50", countValuesGreaterThanFifty[indexToTest], 50L, 100L);
        
        assertNoNestedDocuments("from " + alias, docsCount * 2, 0L, 100L);
        
        assertNoNestedDocuments("from " + alias + " | where data >= 50", Arrays.stream(countValuesGreaterThanFifty).sum(), 50L, 100L);
    }
    public void testGroupingMultiValueByOrdinals() {
        String indexName = "test-ordinals";
        assertAcked(client().admin().indices().prepareCreate(indexName).setMapping("kw", "type=keyword", "v", "type=long").get());
        int numDocs = randomIntBetween(10, 200);
        for (int i = 0; i < numDocs; i++) {
            Map<String, Object> source = new HashMap<>();