Cosmos: Add a way to represent an undefined Partition Key #2586
Description
We currently support specifying a null value for a partition key in the Cosmos SDK, but have no way to express an undefined partition key. The undefined partition key identifies the partition containing items with no partition key property at all.
In the .NET SDK I believe we call this value None, but we should consider other names, since we already use EMPTY to refer to an empty set of partition keys when making a query (indicating a cross partition query)