Allow setting Fingerprint terms to null

[thiemowmde]

Usually I would avoid "magical" behavior like this. For example, this must not be done in the Map base class. It should be possible to store null in a map. A generic map should not have such a magical value.

But labels, descriptions and aliases do have a specific type.

And most important: The getters return null. The setters should accept everything the getter returns.

[mariushoch]

Sounds sensible to me and the implementation looks good. Would like another opinion, though.