Sorry, I'm not aware of such option in Load statement, however using intermediate table let's say testa_tmp, could be an alternative.
Load all data into testa_tmp, then copy via insert/select in a way to apply Default to null values:
1- insert into testa (a, c) select a, c from testa_tmp S where S.b = null ;
So for all rows inserted, b column value will be 'Def' because not specified.
2- insert into testa select * from testa_tmp S where S.b <> null ;
All rows inserted have non-null b value .
3- Drop table testa_tmp