[DocDB] NULL tuples don't show up in unique secondary index scans in PgRegrIndex

[tanujnay112]

Jira Link: DB-6846

Description

Starting from commit 2bee5cf or after it appears that we are failing PgRegressIndex on the following set of queries:

CREATE TABLE usc_asc(k int, v int);
CREATE UNIQUE INDEX ON usc_asc(v ASC NULLS FIRST);
INSERT INTO usc_asc VALUES (44, NULL),(22, 20),(33, 30),(11, 10),(44, NULL);
SELECT * FROM usc_asc ORDER BY v DESC NULLS LAST;

On master, this query still yields the following incomplete result:

 k  | v  
----+----
 33 | 30
 22 | 20
 11 | 10
(3 rows)

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