SQL supports COALESCE

Picodata follow-up

https://www.postgresql.org/docs/current/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL

• Follow-up for picodata!1425 (merged)
• See also sbroad!552 (merged)
• Doc pages to change: https://docs.picodata.io/picodata/{...}