plugins: fix shutdown notification to contain object.

We fixed the others.  There are no fields, but this keeps it consistent.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `shutdown` notification contains `shutdown` object (notification consistency)

doc/PLUGINS.md

@@ -929,6 +929,12 @@ logging or notifications. New rpc calls will fail with error code -5 and (plugin
 responses will be ignored. Because lightningd can crash or be killed, a plugin cannot
 rely on the shutdown notification always been send.
 
+```json
+{
+    "shutdown": {
+    }
+}
+```
 
 ## Hooks
 

lightningd/notification.c

@@ -700,6 +700,9 @@ bool notify_plugin_shutdown(struct lightningd *ld, struct plugin *p)
 	struct jsonrpc_notification *n =
 		jsonrpc_notification_start(NULL, "shutdown");
 
+	/* Even shutdown should follow the same "object inside notification" pattern */
+	json_object_start(n->stream, "shutdown");
+	json_object_end(n->stream);
 	jsonrpc_notification_end(n);
 	return plugin_single_notify(p, take(n));
 }

tests/test_plugin.py

@@ -4266,7 +4266,7 @@ def test_all_subscription(node_factory, directory):
                    "balance_snapshot: {'balance_snapshot': {'node_id': ",
                    "connect: {'connect': {'id': ",
                    "channel_state_changed: {'channel_state_changed': {'peer_id': ",
-                   "shutdown: {}"):
+                   "shutdown: {'shutdown': {}"):
         assert l1.daemon.is_in_log(f".*plugin-all_notifications.py: notification {notstr}.*")
 
     for notstr in ('block_added: ',