Consistently set all fields in pg_stat_replication to null instead of 0

Previously the "sent" field would be set to 0 and all other xlog
pointers be set to NULL if there were no valid values (such as when
in a backup sending walsender).

src/backend/replication/walsender.c

@@ -2826,6 +2826,9 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
 		else
 		{
 			values[1] = CStringGetTextDatum(WalSndGetStateString(state));
+
+			if (XLogRecPtrIsInvalid(sentPtr))
+				nulls[2] = true;
 			values[2] = LSNGetDatum(sentPtr);
 
 			if (XLogRecPtrIsInvalid(write))