PG17 compatibility: Fix -1/Null diff in attstattarget test output (#7749)

Changed `attstattarget` in `pg_attribute` to use `NullableDatum`,
allowing null representation for default statistics target in PostgreSQL
17.

Relevant PG commit:
6a004f1be87d34cfe51acf2fe2552d2b08a79273
6a004f1be8

```diff
 -- 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)
```

src/test/regress/expected/alter_index.out

@@ -32,29 +32,33 @@ SELECT create_distributed_table('t2','a');
 (1 row)
 
 -- verify statistics is set
-SELECT c.relname, a.attstattarget
+-- pg17 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
+SELECT c.relname,
+       CASE WHEN a.attstattarget = -1 THEN NULL ELSE a.attstattarget END AS 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)
 
 \c - - - :worker_1_port
-SELECT c.relname, a.attstattarget
+SELECT c.relname,
+       CASE WHEN a.attstattarget = -1 THEN NULL ELSE a.attstattarget END AS attstattarget
 FROM pg_attribute a
 JOIN pg_class c ON a.attrelid = c.oid AND c.relname SIMILAR TO 'test\_idx%\_\d%'
 ORDER BY c.relname, a.attnum;
      relname      | attstattarget
 ---------------------------------------------------------------------
- test_idx2_980004 |            -1
+ test_idx2_980004 |
  test_idx2_980004 |         10000
  test_idx2_980004 |          3737
- test_idx2_980006 |            -1
+ test_idx2_980006 |
  test_idx2_980006 |         10000
  test_idx2_980006 |          3737
  test_idx_980000  |          4646

src/test/regress/sql/alter_index.sql

@@ -23,13 +23,17 @@ ALTER INDEX test_idx2 ALTER COLUMN 2 SET STATISTICS 99999;
 SELECT create_distributed_table('t2','a');
 
 -- verify statistics is set
-SELECT c.relname, a.attstattarget
+-- pg17 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
+SELECT c.relname,
+       CASE WHEN a.attstattarget = -1 THEN NULL ELSE a.attstattarget END AS 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;
 
 \c - - - :worker_1_port
-SELECT c.relname, a.attstattarget
+SELECT c.relname,
+       CASE WHEN a.attstattarget = -1 THEN NULL ELSE a.attstattarget END AS attstattarget
 FROM pg_attribute a
 JOIN pg_class c ON a.attrelid = c.oid AND c.relname SIMILAR TO 'test\_idx%\_\d%'
 ORDER BY c.relname, a.attnum;