Problem: when importing metadata, a3m METS includes empty fields in dmdSec

[sallain]

Describe the bug
When using a CSV file to import metadata alongside a transfer, the CSV may contain empty fields. For example:

filename	dc.title	dc.subject	dc.subject	dc.subject
objects/birds.mp3	14000 Caen, France - Bird in my garden	field recording	phonography	sound art
objects/beihai.tif	Beihai, Guanxi, China, 1988	satellite imagery

The second item in this table only has one subject term, but because the first one has three subjects, the second has two blank fields.

When a3m imports this into the METS dmdSec, the empty fields are recorded as well:

          <dc:title>Beihai, Guanxi, China, 1988</dc:title>
          <dc:subject>satellite imagery</dc:subject>
          <dc:subject></dc:subject>
          <dc:subject></dc:subject>

In Archivematica, the empty fields are not included in the METS:

          <dc:title>Beihai, Guanxi, China, 1988</dc:title>
          <dc:subject>satellite imagery</dc:subject>

To Reproduce
Steps to reproduce the behavior:

1. Start a transfer in Enduro using the DemoTransfersCSV package from archivematica-sampledata
2. Review the dmdSec for beihai.tif (or another image; birds.mp3 has the most subjects, so any other item in the transfer will have blank subjects)

Expected behavior
Blank fields are not recorded in the METS.

Additional context
This comparison is to Archivematica 1.14. This change was introduced in Archivematica 1.14 through artefactual/archivematica#1813.

This issue was originally filed as artefactual-sdps/enduro#610 but I should've filed it here.