Add Maybe.fromNullable for null -> Empty

I consider it a major oversight that I failed to address Java interop
where `null` is a possibility in my original implementation of Maybe. This
commit adds `Maybe.fromNullable` which works similarly to `Option.apply`. If it is
passed `null` it returns `Empty` but otherwise just wraps a value in
`Just`.

The name `fromNullable` was chosen to be explicit about the fact that it
accepts a null input. It was determined that `Maybe.apply` would be a
bad choice because it might be expected to be the same as Maybe's
`point` (which it is not).