Avoid different-size pointer-to-integer cast

Buildfarm member mamba is unhappy that I wrote "(Datum) NULL" in commit
28d534e2ae:
  https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mamba&dt=2026-04-07%2005%3A08%3A08
Use "(Datum) 0" which is what we do everywhere else.

Discussion: https://postgr.es/m/CANWCAZaOs_+WPH13ow33Q==+FwBwVZkqzm4vND=WEB4_NBmv1Q@mail.gmail.com

src/backend/commands/repack.c

@@ -2943,7 +2943,7 @@ initialize_change_context(ChangeContext *chgcxt,
 			ScanKeyInit(entry,
 						i + 1,
 						BTEqualStrategyNumber, opcode,
-						(Datum) NULL);
+						(Datum) 0);
 			entry->sk_collation = chgcxt->cc_ident_index->rd_indcollation[i];
 		}
 	}