Add possibility to create a proxy custom node not from XML as well

There already exists the functionality to create a proxy custom node in the Dynamo. It has been possible to do  only from XML during uploading a file.

Proxy custom node is a custom node without its own definition (without a group of nodes with connectors between them). It's needed to not throw an exception when there is no definition for custom node and to let a user upload custom node definition later.