[Bug] Chrome makes empty null request

[martijnb92]

A bug was introduced in 0085ee3
If no icon is specified an empty 'icon' attribute is created. This causes Chrome to make a request to serviceworkerdomain/null, which returns a 404 in my case.

I solved this in my service worker by deleting the empty attribute.

      sendNotification (data) {
        delete data.actions[0].icon;
        return self.registration.showNotification(data.title, data)
      },

Attribute shouldn't exist if it is empty.