- flag = datetime:isNull()
 - flag = datetime:isValid()
 
SYNOPSIS
DESCRIPTION
datetime:isNull() checks if datetime is null (0000-00-00T00:00:00), which is also an invalid date/time.
date:isValid() checks if datetime is a valid value.
RETURN VALUE
    A boolean indicating the success of the check.
  
   ERRORS
    Raises an error if datetime is not a datetime instance.
  
   EXAMPLE
> 
> 
      dt = clock.newDateTime()
print(dt:isNull(), dt:isValid())
     true    false
    SEE ALSO
clock.currentDateTime, clock.newDateTime, clock.setDateTime, clock.tzget, clock.tzset, datetime(), datetime:addDays, datetime:addMonths, datetime:addSecs, datetime:addYears, datetime:base90, datetime:toTime_t, datetime:date, datetime:day, datetime:daysTo, datetime:format, datetime:hour, datetime:isdst, datetime:minute, datetime:month, datetime:msec, datetime:offset, datetime:second, datetime:secsTo, datetime:set, datetime:setBase90, datetime:setTime_t, datetime:time, datetime:toLocal, datetime:toUTC, datetime:year, datetime:zone, tostring(datetime)