Bug 866690 - [Dialer] Call Log broken due to mcc being null if no SIM.

If we start the device with no SIM, the Call Log does start correctly since mozMobileConnection.voice.network.mcc is null and the toString() function is invoked on it -> https://github.com/mozilla-b2g/gaia/blob/master/apps/communications/dialer/js/recents.js#L168

The problem is due to mcc being a string and not a number anymore. No need to call .toString() on it.