Remove EmptyArrayRef

Remove EmptyArrayRef

Make it more aligned with std::span.
Almost everywhere it wasn't needed anyhow.
Only exception: ternary conditional operator. But there the type
was already explict in all but one case and that one case
(src/gromacs/domdec/distribute.cpp) was confusing because of
multiple implicit conversions.

Related #2859

Change-Id: I0b61abdc2e60285a7ca17e3bdc7e53cb72c5cf6a