Support more missing() sort options

We could introduce two more options:

• consider the missing values as the greatest/farthest values of the result set, putting it last in ascending mode or first in descending mode

• consider the missing values as the smallest/nearest values of the result set, putting it first in ascending mode or last in descending mode

As a workaround the user can now simulate these two options just using missing().use(…) and providing there a very large/small value. Or using missing().last() or missing().first() depending on the sort order.