Export CSVs have some fields always in header even if no data in export

[wickr]

Descriptive summary

@sarahfish07 noticed our CSV exports contain some fields/columns even if they have no relevant data in the export:

first_line first_line_chorus full_text (may need to be added to bulkrax reserved fields) gps_latitude gps_longitude identification_verification_status military_highest_rank object_orientation original_filename photograph_orientation resolution specimen_type

The fields that are marked in the crosswalk.yml as multiple: false then don't get the split value in the field mapping. Then the CSV processor includes those fields whether there is data or not. Field mapping sample from scarc-test exporter (id: 14)

first_line {"from"=>["http://opaquenamespace.org/ns/sheetmusic_firstLine"]}
first_line_chorus {"from"=>["http://opaquenamespace.org/ns/sheetmusic_firstLineChorus"]}
folder_name {"from"=>["http://opaquenamespace.org/ns/folderName"], "split"=>"\\|"}
folder_number {"from"=>["http://opaquenamespace.org/ns/folderNumber"], "split"=>"\\|"}
form_of_work {"from"=>["http://rdaregistry.info/Elements/w/P10004"], "split"=>"\\|"}
former_owner {"from"=>["http://id.loc.gov/vocabulary/relators/fmo"], "split"=>"\\|"}
full_size_download_allowed {"from"=>["http://opaquenamespace.org/ns/fullSizeDownloadAllowed"]}
full_text {"from"=>["http://opaquenamespace.org/ns/fullText"]}
genus {"from"=>["http://rs.tdwg.org/dwc/terms/genus"], "split"=>"\\|"}
gps_latitude {"from"=>["http://www.w3.org/2003/12/exif/ns#gpsLatitude"]}
gps_longitude {"from"=>["http://www.w3.org/2003/12/exif/ns#gpsLongitude"]}
has_finding_aid {"from"=>["http://lod.xdams.org/reload/oad/has_findingAid"], "split"=>"\\|"}

Expected behavior

Export CSVs only contain headers and columns when data is present in the export.

Related work

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[wickr]

QA Steps:

1. Create a new Exporter, CSV format
2. Find existing content, can limit to 3 or 5 works
3. Inspect result CSV file, confirm fields/columns above aren't included any more

[sarahfish07]

Exported cw-furlong collection (28 works). Blank fields are still included in the export.

[wickr]

QA pass. (initial QA was on prod site only, not staging)

Created 2 exporters on staging: https://staging.oregondigital.org/exporters/14?locale=en https://staging.oregondigital.org/exporters/15?locale=en

Both CSVs had only columns that contained data. folder_number appeared correctly in one exported CSV but not the other.