Insert trigger function: If query affects no rows then return `null`
Insert trigger function is modified to have
`IF r IS NULL THEN RETURN null; ELSE RETURN r; END IF;` in return statement.
* fix primary key changing on upsert, fix#342
* add 'update_columns' in 'on_conflict' object, consider 'allowUpsert'
* 'ConflictCtx' type should respect upsert cases
* validation for not null fields in an object
