Deserializing a region should get back null identifiers as-is

I was seeing NPEs when passing a region like ``new Region("my region", Identifier.parse("someuuidlikecode"), null, null)`` to  ``BeaconManager.startRangingBeaconsInRegion(region)``. I don't have the traceback at hand but it was happening in:

- Identifier.equals
- Region.matchesBeacon
- BeaconService.matchingRegions
- BeaconService.processBeaconFromScan

The problem was that the second identifier of the region was deserialized as an ``Identifier`` with a null ``mStringValue``.