[DocDB] Receiving a null range condition results in a crash in DocRowwiseIterator

[tanujnay112]

Jira Link: [DB-311](https://yugabyte.atlassian.net/browse/DB-311)

Description

It appears that receiving a condition involving NULL at the DocDB layer from YCQL or YSQL improperly causes a kTombstone value to be set as a scan bound for HybridScanChoices, causing a crash when we attempt to append this to a DocKey. The following is a repro of the issue at hand in YCQL:

CREATE KEYSPACE sample_key;
USE SAMPLE_KEY;
CREATE TABLE sample (
     h int,
     r1 int,
     r2 int,
     r3 int,
     PRIMARY KEY (h, r1)
 ) WITH CLUSTERING ORDER BY (r1 ASC)
     AND default_time_to_live = 0
     AND transactions = {'enabled': 'true'};
CREATE INDEX sample_index ON sample_key.sample (r1, r2, r3, h)
    WITH CLUSTERING ORDER BY (r2 ASC, r3 ASC, h asc)
    AND transactions = {'enabled': 'true'};
INSERT INTO sample(h,r1,r2,r3) VALUES (1,2,null,5);
SELECT * FROM sample WHERE r1 = 2 and r2 = NULL and r3 IN (5,55);

The final SELECT up above results in a tserver crash as we improperly convert the condition on r2 into a kTombstone value here and then we attempt to later append this to a DocKey during iteration causing a crash here.