On my computers (22.04, 24.04 and 24.10) the file /usr/bin/notify-send is not empty. I think something went sideways on your system.
Try this on the commandline:
notify-send summary body
If that does not work (which I assume because on your system it is empty) you might want to see if it is installed:
apt search libnotify-bin
And perhaps reinstall it
sudo apt install --reinstall libnotify-bin
The service that you want to create will not work because:
- /usr/bin/notify-send can not be used as a service c.q. daemon.
- /usr/bin/notify-send on your system seems to be an empty file and will therefore do nothing.