Add enforcement of null for properties marked as write-only

[mraineri]

A recent change in the schema started making use of write-only permissions to show when a property is expected to return as null when it represents sensitive data (such as a password). The property itself is in the response so a client understands they're allowed to request a change to it, but the data is not shown. Previously this was only called out in the long description, but now that we're using this permission for the property, we can test for this.