public Instant asInstant() {
        requireType(Type.STRING);
        try {
            return Instant.parse(asString());
        } catch (DateTimeParseException e) {
            throw new InvalidJsonException("Expected JSON member '%s' to be a valid timestamp: %s".formatted(path, e.getMessage()));
        }
    }
@return true if the JSON field was present but its value was 'null'
vespajlib/src/main/java/ai/vespa/json/Json.java