Make tree values iterable without requiring search for empty key.

[raphw]

I am not sure if you wanted to add this, but with these changes all trees become iterable over their values. This can be rather convenient, especially with newer Java versions where tree contents are stream processed.

You can already achieve this by searching for an empty string as a key, but I felt like this would make using code more readable with a minor performance benefit. Please consider this as a suggestion, I fully understand if you did not want to add this, too. Thanks!