Fix FINISH_AT management in Scheduler.

Summary:
Check light frame presence when opening sequence files.
Remove light frame check from imaging duration estimation.
Fix unexpected completion issue on FINISH_AT, which is now obvious.
Fix completion state on FINISH_AT causing a crash.

Test Plan: Tested with various combinations of FINISH_AT jobs.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D14383