Cosmos: consider warning/error when attempting to save and a partition key is unset

[roji]

With #34180, we started warning when an entity is saved and its key property is unset (i.e. the CLR default). The rationale was that it's unlikely that the user actually wants to insert a document with a 0 id property, and likely is expecting the property to be generated like in relational, which it isn't.

We could do the same for partition key properties: if a partition key property is unset, it's quite likely to be an omission rather than actual valid value.