MPASSIT crashes with unhelpful segfault with either interp_* option is true but the corresponding file is empty

[LarissaReames-NOAA]

If interp_diag is true but diaglist is empty, the code fails on the first FieldBundleRegridStore call with unhelpful ESMC errors. Put in checks in initial reading of *list files to make sure that this isn't the case and provide useful errors if it is.