With the latest release to Team Services Kanban Card has been added with a new feature. Previously, if you want to edit a value in a field that is currently empty, in the Kanban Card You had to keep it showing even if value is empty. With the new feature available now you can let Kanban Card to hide a field, if it is empty and expand the Card to show empty fields so that you can edit them.
This will hide a field if it has empty value, even if you have selected the field to show in Kanban Card.
Once you expand you can now see the selected fields for the card even if they currently have empty values. You can edit the fields on the Card and you can collapse the card using the chevron, to hide the empty value fields.