Skip annotation field if empty in filter_non_productive

For example, fields like ``vj_inframe`` etc will be missing in pure Vidjil output. One can of course re-assign them based on ``junction_aa``. Yet the fields ``productive`` is only computed by Igblast. Consider that if a value is missing then it should be not considered, so for Vidjil output filter_non_productive just returns an entire table.