conversion:Only_if

conversion:Only_if is a conversion:Enhancement.

name,length
Susy,0
Johny,10

We can avoid asserting a boat length of 0 with interpret null:

<http://localhost/source/lebot/dataset/dead-end-implicit-bundles-with-interpret-null/version/2013-Jul-30/Susy>
   foaf:name "Susy" ;
   e1:owns implicit_owns:boat_2 ;
   ov:csvRow "2"^^xsd:integer .
   
<http://localhost/source/lebot/dataset/dead-end-implicit-bundles-with-interpret-null/version/2013-Jul-30/Johny>
   foaf:name "Johny" ;
   e1:owns implicit_owns:boat_3 ;
   ov:csvRow "3"^^xsd:integer . 

implicit_owns:boat_2 a local_vocab:Boat .

implicit_owns:boat_3 a local_vocab:Boat ;
   e1:length "10"^^xsd:integer .