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.
### Description
What component does this PR affect?
- [x] Server
### Related Issue
#494
### Solution and Design
Use `quote_ident()` SQL function over `constraint_name` in insert trigger function definition.
### Type
- [x] Bug fix (non-breaking change which fixes an issue)
* 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
