Cherry-pick: Omit DEFAULT NULL when column-default is :NULL (#1609)

## Upstream PR
https://github.com/dimitri/pgloader/pull/1609

## Summary
DDL generation already omits `DEFAULT NULL` when `column-default` is NIL, but not when it's the keyword `:NULL`. Fix adds `(not (eq (column-default column) :NULL))` check.

## Files
- `src/pgsql/pgsql-ddl.lisp`

## Notes
- Low risk, 1-line change
- **Priority: Medium**