fix: ensure null values cast to varchar/string remain null

[stevenpyzhang]

Description

Fixes #4892
String.valueOf() will return the string "null" when a null value is passed in. Using Object.toString(obj, defaultValue) allows the null value to be returned when a null is passed in https://docs.oracle.com/javase/8/docs/api/java/util/Objects.html#toString-java.lang.Object-java.lang.String-

Testing done

Tested the scenario described in the ticket
Added new QTT case

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[agavra]

The fix LGTM, but I think it's backwards incompatible.

[agavra]

looking at the plan.json that was generated from the test, I'm not sure this is backwards compatible - unless I'm missing something, we don't add the generated java code to the plan that we enqueue (cc @rodesai to comment on whether or not including that might be a good idea). That would mean that we would be changing any existing query to have this behavior.

we should consider whether this backwards incompatible change is acceptable or if we should guard this with a config that defaults to disabled, but any new query would have it enabled

[stevenpyzhang]

Added a config for preserving old query behavior.
Testing:

-- On master
SET 'auto.offset.reset'='earliest';
CREATE STREAM HAS_NULL_VALUES (USER VARCHAR, VIEWTIME INT) WITH (KAFKA_TOPIC='test', VALUE_FORMAT='DELIMITED');

CREATE TABLE COUNT_WITH_CAST_NULL_TOGGLE_OFF AS
SELECT USER, COUNT(CAST(VIEWTIME AS VARCHAR)) FROM HAS_NULL_VALUES GROUP BY USER;

-- On branch with fix
SET 'auto.offset.reset'='earliest';
CREATE TABLE COUNT_WITH_CAST_NULL_TOGGLE_ON AS
SELECT USER, COUNT(CAST(VIEWTIME AS VARCHAR)) FROM HAS_NULL_VALUES GROUP BY USER;

-- insert some data into it with null values
INSERT INTO HAS_NULL_VALUES (USER) VALUES ('amy');
INSERT INTO HAS_NULL_VALUES (USER, VIEWTIME) VALUES ('amy', 123);

SELECT * FROM COUNT_WITH_CAST_NULL_TOGGLE_OFF EMIT CHANGES LIMIT 1;
+---------------------+---------------------+---------------------+---------------------+
|ROWTIME              |ROWKEY               |USER                 |KSQL_COL_1           |
+---------------------+---------------------+---------------------+---------------------+
|1585140737589        |amy                  |amy                  |2                    |

SELECT * FROM COUNT_WITH_CAST_NULL_TOGGLE_ON EMIT CHANGES LIMIT 1;

+---------------------+---------------------+---------------------+---------------------+
|ROWTIME              |ROWKEY               |USER                 |KSQL_COL_1           |
+---------------------+---------------------+---------------------+---------------------+
|1585140737589        |amy                  |amy                  |1                   |

[agavra]

LGTM! Only nit is that I don't love the config name, it's a little cryptic