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

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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.