Add type annotation for Optional.empty
Add type annotation for Optional.empty

With sbt 1.4.x, the type is inferred as Optional[Nothing] rather than
Optional[ClassFileManager].

playframework/playframework,playframework/playframework,mkurz/playframework,mkurz/playframework,marcospereira/playframework,marcospereira/playframework,mkurz/playframework,marcospereira/playframework,marcospereira/playframework,mkurz/playframework,playframework/playframework