API-alignment bugs with EmptyQuantity

[ckittl]

Currently, the EmptyQuantity is not totally aligned with the concepts of the measurement API. The following issues should be addressed:

• AbstractQuantity#toString fails rendering the null value to a sensible output
• AbstractQuantity#compareTo(Quantity) fails, as here as well, null is not compliant with the Number interface.

This might not be comprehensive, therefore, we should check all methods, that are yet implement in the abstract class.