PG17 compatibility: Fix Test Failure in alter_index

[m3hm3t]

Changed attstattarget in pg_attribute to use NullableDatum, allowing null representation for default statistics target in PostgreSQL 17.
postgres/postgres@6a004f1

 -- verify statistics is set
 SELECT c.relname, a.attstattarget
 FROM pg_attribute a
 JOIN pg_class c ON a.attrelid = c.oid AND c.relname LIKE 'test\_idx%'
 ORDER BY c.relname, a.attnum;
   relname  | attstattarget 
 -----------+---------------
  test_idx  |          4646
- test_idx2 |            -1
+ test_idx2 |              
  test_idx2 |         10000
  test_idx2 |          3737
 (4 rows)

[m3hm3t]

old pr:
#7729