ItemsTransformer shouldn't require call number configuration

Most migration don't map call numbers at the item level. However, the process won't run unless call_number_type_mapping.tsv is present, it contains a valid column header from the item file, as well as a valid call number type even though no call number field is read.

I can't say I've _ever_ mapped a call number type from an item file. In the rare cases where I've mapped call numbers, there was no way to map the call type