private Map<String, Object> getUnmodifiableFieldMap() {
        if (unmodifiableFieldMap == null) {
            if (fields == null) {
                return Map.of();
            } else {
                unmodifiableFieldMap = Collections.unmodifiableMap(fields);
            }
        }
        return unmodifiableFieldMap;
    }
Returns whether any fields are set in this
container-search/src/main/java/com/yahoo/search/result/Hit.java