Fix task_edit.py when there is not yet subproblems in a task
Fix task_edit.py when there is not yet subproblems in a task

GuillaumeDerval/INGInious,GuillaumeDerval/INGInious,layus/INGInious,GuillaumeDerval/INGInious,layus/INGInious,layus/INGInious,layus/INGInious,GuillaumeDerval/INGInious