PUGH: support zero sized grid arrays

if a grid array has size 0 we ignore it during communication by setting its ghost size and overlap region to zero, cctk_lsh will show up a zero as well

This fixes the bug that Erik observed in https://bitbucket.org/cactuscode/cactustest/commits/8a6601f about 14 years ago. As far as I can tell this test has been failing since then and only was not caught b/c the testing system actually does not look at the Cactus return code.