sql: add no-op {sh,}obj_description builtin
sql: add no-op {sh,}obj_description builtin

These builtins retrieve comments from database objects. We don't support
commenting on database objects, so they both return null.

paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach,paperstreet/cockroach