Bug with writing empty global attrs

@bryanharter  We are seeing an issue where if an xarray.Dataset has a global attribute set to None, the global attributes that follow that one don't get written to the CDF. Is this expected behavior? I think it is this if statement with the return in the write_globalattrs() function: https://github.com/MAVENSDC/cdflib/blob/main/cdflib/cdfwrite.py#L433