Fixed 404 on courseware when users last visited subsection becomes unpublished.

"Page not found" if student visits courseware after the section they visited becomes unpublished.
LMS-6492
@adampalay Because when a subsection have only one vertical in it and this subsection is a first subsection of the course, suppose a student visited this vertical and it becomes his last visited problem, now he signed out after that course staff changed this problem release date to future or unpublished, now student came back and clicked on courseware then last-visited state not found and we cleared his last-visited state and redirected him to courseware again to load first time view but again it comes to first subsection which have no item in it now and it returns 404 thats why i added this condition to get only those subsections which have a children in it.