openlayers/ol3 — pull request #1510 by tschaub (2014-01-10 20:53:48+00:00)
Title: 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.