use `weak_ptr`'s `lock()` method to allow case of no coarser level

On the coarsest level, element_->coarser_ is empty and constructing a shared_ptr from a weak_ptr fails. The lock() method returns an empty shared_ptr instead.

This fails when iterating over the hierarchy as the end of the iteration cannot be reached; the problem was previously described by @simon.praetorius 1.

changed milestone to %DUNE 2.7.0

added bug label

mentioned in commit 01b26888

merged