Portlet import/export does not work if you empty a portlet manager

If one creates a classic site and removes the left and right portlets, the portlet exporter doesn't signal that those columns are empty:

This is an example portlets.json file removing the left and right columns, and removing all other footer portlets except the site actions:

[
  {
    "@id": "http://nohost/Plone",
    "UID": "plone_site_root",
    "portlets": {
      "plone.footerportlets": [
        {
          "assignment": {
            "category": "site_actions",
            "default_icon": "action_icon.png",
            "ptitle": "site_actions",
            "show_icons": false,
            "show_title": false
          },
          "type": "portlets.Actions",
          "visible": true
        }
      ]
    }
  }
]

Thus, if you import the created json files into a site, the default left and right portlets are kept.

Start by tracing the portlet export and import handling in plone.exportimport, using the example portlets.json as the reproduction case. Verify that removing all portlets from the left and right managers is represented during export and preserved after import.