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).