orm: Propagate LEFT joins in join paths

If something like members__staff is considered leaving the Team model,
and the `members` relationship is nullable, and the `staff` relationship is
not, in the context of the compiled SQL statement, the second join should
also be considered nullable (LEFT join), because otherwise inconsistent
results would be returned from the query.

In other words, if a count is considered as an annotation to the Team model
instances, Teams with zero members should still be considered as valid teams
and should be selected with such an annotation. Before this patch, however,
the join between TeamMember and Staff would have been an inner join instead
of a LEFT join, which could skew the database results.