`ScalaPresentationCompiler.apply` should never return `Some(null)`

The former implementation of `ScalaPresentationCompiler.apply` could return
`Some(null)` if the passed `op` returned `null`. This was not the intended
semantic.

A different issue I'm thinking about is how I can provide an overload of
`ScalaPresentationCompiler.apply` to allow to pass either an `op` of type
`ScalaPresentationCompiler => U` or `ScalaPresentationCompiler => Option[U]`.
The simplest way to do so would be to have two different methods, but I want to
think about this a bit more.

By the way, this fix is needed to make the play2 support work with the latest master (will create a PR shortly, and a test will be failing until this gets merged).