- flag = time:isNull()
 - flag = time:isValid()
 
SYNOPSIS
DESCRIPTION
time:isNull() checks if time is null (midnight = 00:00:00.000).
time:isValid() checks if time is in the valid range of 00:00:00.000 and 23:59:59.999.
RETURN VALUE
    A boolean indicating the success of the check.
  
   ERRORS
    Raises an error if time is not a time instance.
  
   EXAMPLE
> 
> 
> 
      t1 = clock.newTime()
t2 = clock.newTime(99)
print(t1:isNull(), t1:isValid(), t2:isValid())
     true    true    false