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. https://github.com/postgres/postgres/commit/6a004f1be87d34cfe51acf2fe2552d2b08a79273

 -- 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: https://github.com/citusdata/citus/pull/7729

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```diff @@ Coverage Diff @@ ## release-13.0 #7749 +/- ## =============================================== Coverage ? 89.64% =============================================== Files ? 274 Lines ? 59583 Branches ? 7436 =============================================== Hits ? 53416 Misses ? 4035 Partials ? 2132 ```