Fix a bug for maps with zero entries
Description
This case can happen in case of very thick boundary layers and many tasks the grid is distributed over. Then some might not have any inner points, leading to a call to the copy_with_map function with 0 points. This lead to an error in the cuda functions as the corresponding map array never got allocated on the GPU.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.