fix(cubesql): Generate typed null literals #3639

Job	Run time
latest-tag-sha	20s
Build native Linux 20 x86_64-unknown-linux-gnu Python fallback	2m 55s
build	7m 25s
tests (20.x, athena-export-bucket-s3)	18m 56s
tests (20.x, bigquery-export-bucket-gcs)	8m 14s
tests (20.x, clickhouse)	5m 58s
tests (20.x, clickhouse-export-bucket-s3)	5m 26s
tests (20.x, databricks-jdbc)	7m 6s
tests (20.x, databricks-jdbc-export-bucket-s3)	7m 5s
tests (20.x, databricks-jdbc-export-bucket-azure)	6m 38s
tests (20.x, mssql)	5m 55s
tests (20.x, mysql)	5m 18s
tests (20.x, postgres)	5m 26s
tests (20.x, redshift)	6m 51s
tests (20.x, redshift-export-bucket-s3)	5m 36s
tests (20.x, snowflake)	5m 37s
tests (20.x, snowflake-export-bucket-s3)	5m 43s
tests (20.x, snowflake-export-bucket-azure)	5m 47s
tests (20.x, snowflake-export-bucket-azure-via-storage-integration)	5m 46s
tests (20.x, snowflake-export-bucket-gcs)	5m 56s
	2h 7m 58s

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