Fix DofMap::collapse() on rank with no dofs

[jorgensd]

Currently, collapsing a dofmap with no dofs on the process segfaults due to unsafe access of dofs_view.back(), as it might be empty.

This is unlikely to happen to "standard" meshes, but very likely to happen for sub-meshes.

Thanks to @RemDelaporteMathurin for providing me with a segfaulting code that I could distill down.