Opening traffic splitting modal when there are no revisions breaks the UI

Description of problem:

 Opening traffic splitting modal when there are no revisions breaks the UI.

Prerequisites (if any, like setup, operators/versions):

Openshift serverless operator should be installed.

Steps to Reproduce

1. Create an application using Knativ serving.
2. Right click on the service in topology.
3. Click `Set traffic distribution`

Actual results:

UI crashed with an error.

Expected results:

Ui should not crash with an error.

Reproducibility (Always/Intermittent/Only Once): Intermittent

Build Details:

Additional info: