Minor ODATA-24 simplified relationships by moving association information into the navigation property. In doing so, cardinality was represented by the type of the navigation property being either a single entity or a collection of entities. For a single entity, optionality of the entity (i.e., 0 or 1) is represented by a nullable attribute (whose default is false).
For navigation properties that return collections of entities, nullable=true doesn't make sense; the collection always exists, it may just be empty. Thus, collection-valued navigation properties should either:
1) always specify nullable=false
2) explicitly prohibit specifying nullable=true, or
3) disallow the nullable attribute
Note that there is precedence for the set of valid attributes depending on the type (i.e., in specifying property facets).
This doesn't quite address the issue. The point is, it isn't valid to specify nullable=true for a collection valued property; the collection is always there, it may just be empty.
The easiest way to support this would be to require setting nullable=false for collection-valued properties.
Using false as default for Nullable is inconsistent with its default value for "normal" properties. I recommend keeping "true" as the default.
Added the restriction to non-colllection types.