Handle case where resource has no imagery providers

This avoids an Uncaught TypeError: Array.prototype.map called on null or undefined.

See the [bing-maps.html](http://ol3js.org/en/master/examples/bing-maps.html).  It looks like the Collins Bart and Ordnance Survey metadata is returned with `null` `imageryProviders`.  I haven't checked to see if a different attribution source should be used.