Remove empty fields from output

This way the mapper does not have to check if a
field has any content to copy.
2 jobs for 0.2.0 in 6 minutes and 13 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed test #26416

00:02:56

 
  Publish
passed build-tagged-image #26417

00:03:16