xarray does not support use of '_FillValue' for unicode strings

[aburrell]

Describe the bug
to_netcdf uses SPDF ISTP/IACG standards for netCDF files by default, which includes the '_FillValue' variable. xarray currently does not support unicode '_FillValue' inputs. This is documented here: pydata/xarray#1647

Expected behavior
Appropriate '_FillValue' attribute should be set for all variables, regardless of type.

Additional context
The current work around removes _FillValue for xarray unicode variables. Once this is supported, the removal of this attribute should be removed.