[BUG] Workflow fails if `close-reason` is empty

[LukeSavefrogs]

The documentation states that close-reason is optional:

However this is only partially true, since if not provided and close is true, the workflow will fail with the following error:

Error: GraphqlResponseError: Request failed due to following response errors:
 - Variable $reason of type DiscussionCloseReason was provided invalid value

I suggest to update the documentation with something like the following:

-Optional, defaults to ''
+Optional when `close` is set to `false`, defaults to ''