[FIX] project: default stage on task

If you create a task directly, the key `default_project_id` is not set
in the context. This key is used to compute the default stage of the
created task. It is done by creating a domain. Unfortunately, the
domain is always filled with `[('fold', '=', False)]` and thus it
always returns a result that makes no sense when `default_project_id`
is not set. Worse, if you actually create the task, the found stage
that was not in your project is now a part of it.

The fix is to return no result if the `default_project_id` is not set
in the context.