Invalid CASTed timestamps as strings are not seen as "null" by IFNULL.

[jnh5y]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce

CREATE STREAM test (id INT KEY, data STRING) WITH (kafka_topic='test', key_format='json', value_format='json');
// Insert a good record: 
INSERT INTO test VALUES (1, '2022-01-01');
// Insert a bad record: 
INSERT INTO test VALUES (2, 'xxxx');
INSERT INTO test VALUES (3, null);
// Substitute null for parsed timestamp and see the result: 
SELECT  ifnull(cast(PARSE_DATE(null, 'yyyy-MM-dd') as string),'1900-01-01') from test;```

Output:

```+------------------------------------------------------------------------------+
|KSQL_COL_0                                                                    |
+------------------------------------------------------------------------------+
|2022-01-01                                                                    |
|null                                                                          |
|1900-01-01                                                                    |
Query terminated

Expected behavior
Non-null values.

[jnh5y]

@stevenpyzhang suggests a fix like this here:

ksql/ksqldb-execution/src/main/java/io/confluent/ksql/execution/codegen/SqlToJavaVisitor.java

Line 855 in 5413985

public Pair<String, SqlType> visitCast(

String code = genCastCode(expr,to);
      code = evaluateOrReturnNull(code, node.getType().toString());
      return Pair.of(code, to);```