Use noop lambda in dummy NotificationCommand

The dummy command would sometimes fail with a "broken pipe" error
on slow runners like the arm64 container image one, so this commit
replaces them with executing a noop lambda. Using "/bin/echo" might
also have been fine, but with this there isn't even a process spawned
that can get a broken pipe.

test/notification-notificationcomponent.cpp

@@ -52,7 +52,7 @@ object Host "h1" {
 	enable_passive_checks = true
 }
 object NotificationCommand "send" {
-	command = ["true"]
+	execute = {{}}
 }
 apply Notification "n1" to Host {
 	interval = 0