Change return type of ContentTypes.parse.empty BodyParser from Option[Any] to Unit

This BodyParser never parses any content so a result of Unit is simpler to understand
than having a return type of Option[Any] but always returning None.
Expose empty BodyParser in Java API

Using an empty BodyParser can increase Java action performance.
Merge pull request #2613 from richdougherty/empty-bodyparser

Add empty BodyParser to Java API