Void return types need to always be null value

[jonahgraham]

Void type in Java cannot be instantiated, therefore if a type is declared as Void the fromJson should always set that value to null.

If you want to have a generic return type that allows non-null and null results, use a type other than Void as the return type.

Fixes #721

[jonahgraham]

This has recreated #728 but in the eclipse/lsp4j repo so that we can get a p2 site out of it easily.

[jonahgraham]

@sebthom - here you go https://download.eclipse.org/lsp4j/builds/jonah/fixes_721/

[sebthom]

I tested it with the Haxe and Dart Debug Adapters and it works fine.