Descriptive spreadsheet import should treat space-only values as null

[arcadiafalcone]

In the Cocina spreadsheet, a field containing spaces may appear blank to the user, but results in an error that is only raised at indexing. (Most recent instance: https://github.com/sul-dlss/exhibits/issues/2461.) When parsing the spreadsheet, a value consisting only of whitespace characters should be treated the same as if there were no value.