How to return when an optional is empty?

You can use ifPresent and map methods instead, if the function is void and you need to do side-effects you can use ifPresent,

optional.ifPresent(System.out::println);

If another method return relies on the Optional than that method might need to return an Optional as well and use the map method

Optional<Integer> getLength(){
    Optional<String> hi = Optional.of("hi");
    return hi.map(String::length)
}

Most of the time when you call isPresent and get, you are misusing Optional.

You could use orElse(null):

String o = getOptional().orElse(null);
if (o == null) {
    return;
}

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