No tasks for empty cells

Merged Matthieu Schaller requested to merge no_tasks_for_empty_cells into master

In the last big update, I was too enthusiastic about the setting of the super pointers. This lead to empty cells having tasks assigned to them. This breaks one of our runtime example and is fixed by this change.

However, since touching the super-pointer construction is a dangerous affair, some cross-check of what I have done here would be appreciated.

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