DS-2174 Metadata Export to CSV, fix bug with empty language []

https://jira.duraspace.org/browse/DS-2174

Change MetadataExport to not spit out columns such as dc.date.submitted[], instead you will just get dc.date.submitted

I found what I believe is a bug, where if a metadata field had a non-null language value, such as empty string, it would add [] to the metadata field. Sometimes you get [], other times you don't.