Bring `-compare:` implementations inline with macos

[ERobsham]

updates the implementation of -compare: vs a nil argument to return the same results as the same code running on macos (output from a sample program on macos):

NSString 'compare:' nil arg: NSOrderedDescending
NSDate   'compare:' nil arg: NSOrderedSame
NSNumber 'compare:' nil arg: exceptions `-[__NSCFNumber compare:]: nil argument`

Related issue: https://github.com/gnustep/libs-base/issues/507