creating registration with non-configured status throws error

[lcdservices]

to reproduce:

• configure your participant statuses that should be considered when synching with GTW. note a status that is not synched.
• create a registration in the backend for a contact and select a non-synched status.

this throws the error:
Webinar Error : InvalidRequest
Your request could not be processed because the request body is not parsable.

in the post hook when the $fields array is created, we should return immediately if empty (i.e. if there is no matching participant record that meets the criteria, which includes the status).

[lcdservices]

PR for consideration: #49