If a PATCH request is applied to a resource that doesn't exist yet, is it then allowed to create the resource or do I need a separate POST/PUT request in that case?
The PATCH request would go to the URL for the resource for example: PATCH /object/1234. If the object with the ID 1234 is in the database I'll create it, otherwise I'll update it.
The PATCH request doesn't contain all fields, that's why I don't use PUT.
RFC 5789 states that PATCH should be used "to modify an existing HTTP resource." It would probably be best to implement a POST/PUT request in order to adhere to the HTTP standards.