Clarify handling of null values for complex-typed properties

The JSON Format specification describes in section 7.1 Primitive Value how a null value is represented for a property of primitive type. However, an analogous description is not provided in section 7.2 Complex Value. 

The case of complex-typed properties having null values is intended as the URL Conventions specification describes in  5.1.1.1.1 Equals how the eq-operator works for such properties to test against the null value.

Hence it seems to be a gap that section 7.2 of the JSON Format document does not explicitly mention null values.