Clear 'xid' in dummy async notify entries written to fill up pages

Before we started to freeze async notify entries (commit 8eeb4a0),
no one looked at the 'xid' on an entry with invalid 'dboid'. But now
we might actually need to freeze it later. Initialize them with
InvalidTransactionId to begin with, to avoid that work later.

Álvaro pointed this out in review of commit 8eeb4a0, but I forgot
to include this change there.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://www.postgresql.org/message-id/202511071410.52ll56eyixx7@alvherre.pgsql
Backpatch-through: 14

Author: hlinnaka

src/backend/commands/async.c

@@ -1470,6 +1470,7 @@ asyncQueueAddEntries(ListCell *nextNotify)
 			 */
 			qe.length = QUEUE_PAGESIZE - offset;
 			qe.dboid = InvalidOid;
+			qe.xid = InvalidTransactionId;
 			qe.data[0] = '\0';	/* empty channel */
 			qe.data[1] = '\0';	/* empty payload */
 		}