There is the assertion:
"Nullable MUST NOT be specified for a collection-valued navigation property, a collection is allowed to have zero items."

A property, regardless of multiplicity consumes a slot in an instance (entity representation). The slot has a sequence of values, depending on cardinality. If the slot has no values, then the property is said to be null. If an entry in a list of values has no value, then that entry is null. So, you can both have a collection property that is null or a collection property consisting of a list of null values. After much discussion, the DMTF specified the following for the values of a collection property (i.e. upper bound on multiplicity > 1)

• NULL: The collection has no specified value.
• [] : The collection has no elements (I.e. size = 0
• [ Null ] : The collection has one element specified as having no value
• [ "" ] : The collection has one element specified as having the empty string
• [ "x", Null, "y",...] : The collection has multiple elements, some may be specified as having no value.

The collection representing the list of values is not a separate modeled entity, rather it is a representation of the value list of the property itself.