good question, and I was wrong.
as of this latest commit, push notifications are represented by PushNotification types. the notification message is the 4th parameter in the constructor.
this kind of object is created in PushNotificationManager and PushNotificationScheduler.
out of 6 functions that send a notification, 1 actually sends notifications of messages, and it omits putting in the message part: https://github.com/signalapp/Signal-Server/blob/75948dc6174ed091180aa20c018fa0f353130ee0/service/src/main/java/org/whispersystems/textsecuregcm/push/PushNotificationManager.java#L49
so the signal server does not send {"urgent": true} to firebase, it sends no message, just kind of a ping in my understanding.