Support System.Nullable in ProductStorage

[dbeuchler]

Is your feature request related to a problem? Please describe.
My product instances often has properties which are filled during production. For example DateTime, double, or integers.
At the moment I must set them to a default-value or MinValue to be stored. double.NaN is working in PostgreSQL but not in SQLite.

The knowledge about the underlying ProductStorage is essential to store the Types and Instances.

Describe the solution you'd like
Supporting System.Nullable in the ProductStorage would help to minimize unexpected problems.

TODO:

• Change Entities to Nullable-Columns by default
• Support System.Nullable in ProductStorage strategies.