Use LEFT JOIN when building WP_Tax_Query SQL.

`LEFT JOIN` ensures that `NOT EXISTS` queries will not miss posts that have
no taxonomy data whatsoever.

Props swissspidy, crstauf.
Fixes #36343.
Built from https://develop.svn.wordpress.org/trunk@37184


git-svn-id: http://core.svn.wordpress.org/trunk@37151 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-tax-query.php

@@ -440,7 +440,7 @@ class WP_Tax_Query {
 				// Store the alias with this clause, so later siblings can use it.
 				$clause['alias'] = $alias;
 
-				$join .= " INNER JOIN $wpdb->term_relationships";
+				$join .= " LEFT JOIN $wpdb->term_relationships";
 				$join .= $i ? " AS $alias" : '';
 				$join .= " ON ($this->primary_table.$this->primary_id_column = $alias.object_id)";
 			}