Add "NOT NULL" option to targets

Currently Feeds implements one possible option for all targets — UNIQUE.
Why not to go further and add NOT NULL field option? This would allow to skip source records which doesn't conform some criteria.

Simplest case: skip records where specific field is empty.
More intricate case: skip records which rewritten (e.g. by Feeds Tamper) field values is empty.

Implementation seems to be pretty evident. In FeedsProcessor->map() function before settings values with setTargetElement() or with target's callback, check the $value and if it is empty (all elements of the array is empty) then return NULL instead of $target_item. Correspondingly, in process() — just skip importing.

Thoughts?