Rich card without `tap` should not error on render

[compulim]

Rendering a hero card like the one below, should not cause any errors.

{
  "subtitle":  "What do you think of these?",
  "buttons": [
    {
      "type": "openUrl",
      "title": "Not what I'm looking for",
      "value": "http://contosocafe.com/specials"
    },
    {
      "type": "imback",
      "title": "These look great",
      "value": "these look good"
    }
  ]
}