Ensure our scripts don't generate NaN fill values in netCDFs

NaN fill values on netCDFs are problematic for the ifx compiler. Existing input files were fixed in ctsm5.4.022 (PR #3787) and ctsm5.4.023 (PR #3809), but the scripts we used to generate input files presumably still might include NaN fills. They should be changed to avoid this. One annoying factor is that xarray's `Dataset.to_netcdf()` adds NaN fills by default if a variable has no `_FillValue`, requiring the user to include an argument of `encoding={var0: {"_FillValue": None}, {...}, ...}` for every variable where we don't want that.