CellIterator iterates over cells with no DOFs

I've tried out the fix for #966. However, it seems like the CellIterator still iterates over every cell in the Grid instead of only the cells belonging to the DofHandler. When Iterating over a cell that doesn't belong to the DofHandler, the CellCache will be reinitialized with an empty dof vector.

```julia
using Ferrite

grid = generate_grid(Hexahedron, (3,3,3))

dh = DofHandler(grid)
sdh = SubDofHandler(dh, [1,2,3])
add!(sdh, :u, Lagrange{RefHexahedron,1}()^3)
close!(dh)

for cell in CellIterator(sdh) # works
@assert celldofs(cell) |> !isempty
end

for cell in CellIterator(dh, [1,2,3]) # works
@assert celldofs(cell) |> !isempty
end

for cell in CellIterator(dh) # errors
@assert celldofs(cell) |> !isempty
end
```