UX Improvement: Present fields for export in checkboxes instead of multiselect

Problem/Motivation

The UX of multiselect fields is notoriously poor. We had a request to migrate the CSV export form to checkboxes and, ultimately, it did not appear to be that difficult.

Note:

• We originally just switched the fields #type from select to checkboxes, but that introduced a fatal error when submitting the form:
  

  InvalidArgumentException: Field 0 is unknown. in Drupal\Core\Entity\ContentEntityBase->getTranslatedField() (line 616 of core/lib/Drupal/Core/Entity/ContentEntityBase.php).
  
  Drupal\Core\Entity\ContentEntityBase->get() (Line: 267)
  Drupal\content_export_csv\ContentExport->getData() (Line: 205)
  Drupal\content_export_csv\ContentExport->getNodeData() (Line: 108)
  Drupal\content_export_csv\ContentExport->getNodeDataList() (Line: 244)
  Drupal\content_export_csv\ContentExport->getNodeCsvData() (Line: 176)
  Drupal\content_export_csv\Form\ContentExportForm->submitForm()
  

• We introduced a small fix to remove empty fields before invoking getNodeCsvData().