Fix for saving `[NSNull null]` email to NSUserDefaults

Both `emailAddress.length > 0` and `[emailAddress isEqualToString:@""]` also implicitly check for `emailAddress != nil`, which using `emailAddress.length == 0` in the second case, for example, would not check. Kind of a moot point as by this point `emailAddress != nil` has already been checked, but good practice IMO.