Are you confident that conditions for firing the alert are being met and the credentials you're using to execute against the device are correct?
I believe if you're testing the alert, you may find that the script does not execute and you'll get a blank result.
I have an alert to notify when a Cisco device reboots and to execute an NCM action to do a 'show ver'. I've just tested against this and the script doesn't execute, or rather the ${Notes} value doesn't get populated.
If I reboot the same Cisco device, the alert fires and I get the NCM executed action in the ${Notes} field through in an email.
If you're not sure, verify the credentials in the NCM manager (the actual application on the server, not the web console) or run a session trace.
To run a session trace you'll need to turn this on in the NCM web console (Settings > NCM Settings > Advanced Settings and tick 'Enable Session Tracing').
Test or run the alert and then check the C:\Program Files (x86)\SolarWinds\Orion\NCM\Session-Trace folder
Don't forget to turn off session tracing once you've completed your testing!