Flatmap / mapObservable should not handle null values in a special way

[tobiasdiez]

Currently the FlatMapBinding only invokes the mapper if the current value is not null. We should investigate if this is indeed the correct behavior or if we should always call mapper even if the value is null, and use OptionalBinding stuff to handle null values differently.

See also the methods mapObservable and selectProperty in ObservableOptionalValue.