@Override
    public String toString() {
        if (defined) {
            return "DefinedMaybe{" + nullableValue + "}";
        } else {
            return "UndefinedMaybe{}";
        }
    }
A container that may be empty, may contain null, or may contain a value.
Distinct from {@link java.util.Optional<T>}, since Optional cannot contain null.

@param <T>
streams/src/main/java/org/apache/kafka/streams/state/internals/Maybe.java