_adjacent_lesson breaks on persons without lessons

In _adjacent_lesson there is used lessons_on_day which makes use of filter_from_person that can return Ǹone in case that there are no data. lessons_on_day tried to run order_by on None.

added part::backend type::bug labels

changed title from _adjacent_lesson doesn't respect users without lessons to _adjacent_lesson doesn't respect persons without lessons

created merge request !126 (merged) to address this issue

mentioned in merge request !126 (merged)

Please add some explanation to this issue.

changed the description

changed title from _adjacent_lesson doesn't respect persons without lessons to _adjacent_lesson breaks on persons without lessons

closed with merge request !126 (merged)

mentioned in commit d261b968