### Elasticsearch Version

8.8.0-SNAPSHOT

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

MacOS

### Problem Description

API when called is returning invalid response, on the same data terms aggs query runs fine 

![image](https://user-images.githubusercontent.com/3505601/220749805-ddf0d735-60d5-4e45-85bf-941ea3076a3f.png)


When run as aggs query 

![image](https://user-images.githubusercontent.com/3505601/220764360-49f7b648-8b15-43cf-afe2-01bd62e8ebc5.png)


### Steps to Reproduce

Queries are mentioned above ^^

### Logs (if relevant)

_No response_
x-pack/plugin/mapper-constant-keyword/src/main/java/org/elasticsearch/xpack/constantkeyword/mapper/ConstantKeywordFieldMapper.java
@Override
    public SourceLoader.SyntheticFieldLoader syntheticFieldLoader() {
        String value = fieldType().value();
        ;
        if (value == null) {
            return SourceLoader.SyntheticFieldLoader.NOTHING;
        }
        return new SourceLoader.SyntheticFieldLoader() {
            @Override
            public Stream<Map.Entry<String, StoredFieldLoader>> storedFieldLoaders() {
                return Stream.of();
            }
            @Override
            public DocValuesLoader docValuesLoader(LeafReader reader, int[] docIdsInLeaf) {
                return docId -> true;
            }
            @Override
            public boolean hasValue() {
                return true;
            }
            @Override
            public void write(XContentBuilder b) throws IOException {
                if (fieldType().value != null) {
                    b.field(simpleName(), fieldType().value);
                }
            }
        };
    }
}