The default join type for a relationship is an inner join. However, you can create a custom SQL expression, valid as any other table in the Elasticube. You can place a left join statement b/w the two tables, and then use this table for this specific widget. In this Table's expression place the following statement:
FROM tbl_a a LEFT JOIN tbl_b b
ON a.ID = b.ID
For more info on this, please refer to the 5 Simple Rules for Elasticube Design article.