Prevent ArgumentError for 0 dates

[andygeers]

If a DOS Time is 0, it raised an ArgumentError. This fixes it to behave the same as a nil time.

[javanthropus]

The way I solved this was to fix the incorrect method call that was triggered when a nil was passed to the DOSTime initializer. Passing a zero value is considered an attempt to pass a valid structure to the initializer, but this will fail now since that structure contains elements that are out of range.