Fix issue 165: [NSKeyedUnarchiver initForReadingWithData:]: data is NULL on startup

Instead of blindly attempting to read the specified NSData object, check to see if it's nil first.
If it is nil, just return nil (which we were doing anyway) instead of running it through the
NSKeyedUnarchiver class, which was generating a rather disturbing (and irritating) warning.
Merge pull request #166 from aaronbrethorst/issue_165

Fix Issue #165