resolve get on device edge case

[Yurlungur]

PR Summary

@chadmeyer noticed that GetOnDevice on an empty DataBox causes an assertion error related to array sizes. I think this was indicating an edge case in GetOnDevice: the edge case being being that GetOnDevice was attempting to copy memory to device even for unallocated DataBoxes containing no memory.

This PR simply adds a little default code making GetOnDevice a no-op for empty DataBoxes.

PR Checklist

• Code is formatted. (You can use the format_spiner make target.)
• Adds a test for any bugs fixed. Adds tests for new features.
• If preparing for a new release, update the version in cmake.