Fix not blocking / not blocked queries

Summary: Fixes T7434. We need to LEFT JOIN, not JOIN here, because we still want result rows where the value is null.

Test Plan: Issued blocked/not-blocked + blocking/not-blocking queries, got results in all cases.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7434

Differential Revision: https://secure.phabricator.com/D11939