Returns an empty array in chrome.nfc.getDevices when there are no results

So that we're consistent with the _chrome.usb_ API, it would be better to return an empty array instead of null in chrome.nfc.getDevices when there are no results.

See https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/extensions/api/usb/usb_api.cc&sq=package:chromium&type=cs&l=627