Initializing a String on basis on java streams results in an empty String [duplicate]

Question

I have the following code in my example:

final List<Optional<String>> messages = 
    ImmutableList.of(Optional.empty(), Optional.empty() );

final String dummy = messages.stream()
        .filter( Optional::isPresent )
        .map( message -> message.get() )
        .collect( Collectors.joining( "\n---\n" ) );

Now on debugging, the value of the variable dummy is always an empty string. Is that a result of the stream, or the behavior of the Collectors.joining method?

I was under the assumption that the resultant String would be null.