#295, introducing MapNullToDefault

See discussion #295. This new annotation on mapping method level (and new methods in the `@MapperConfig` and `@Mapper`) introduce a new mechanism to determine the output of a mapping method when the source is null.

For beans, an new empty bean is returned (and constants + expressions are populated).
For iterable mappings an empty collection is returned
For map mappings an empty map is returned.

The mechanism is aligned with other hierarchy mechanisms, such as the collection strategy.
But that all depends on how you see constants and expressions. I don't see them as part of the source object. The annotation concerns what to do in case of null source objects.

The 'UUID' case (where I do need to add a UUID to a target even when the source is null) is quite common for me.  Just as the constant. 

What we could to break the `stalemate`  is to introduce an additional enum value.