Allow arrays/collections to be null

[ralfhandl]

Currently collections can only be empty, they cannot be null.

Adding null for collections would allow expressing the subtle difference between "empty wallet" and "no wallet".

Superset of #290

[HeikoTheissen]

This affects collections as

• values of structural properties OData-CSDL, 7.1
• values of navigation properties OData-CSDL, 8.1
• entity sets OData-CSDL, 13.2
• dynamic expressions OData-CSDL, 14.4.6

[HeikoTheissen]

For collection-valued types, Nullable="true" means that the collection can contain null values. A new attribute CollectionNullable="true" would be needed to express that the entire collection may be null.

If an unbound function has two overloads

• <Parameter Name="P" Type="Edm.String" Nullable="true"/>
• <Parameter Name="P" Type="Collection(Edm.String)" Nullable="true" CollectionNullable="true"/>

which overload would then be chosen for the invocation ns.func(P=null)?