API: Fix simplified continuation module skipping

The simplified continuation includes a continuation parameter to
indicate when the prop/list/meta modules are complete, so it can skip
executing them.

But if someone submitted a malformed value for 'continue', it might
decide that the generator was complete but still try to uselessly
execute the prop modules.

Bug: 72764
Change-Id: I6af24e5d4f30e64782fb7dd1c2211ebdd4ec2317
(cherry picked from commit c2e6bdfce71792b97d9eb76e4ffd9b83b1af71af)