Cannot Start Scenario without Ego Vehicle Defined and Spawned

[tlandle]

Describe our question or idea
According to #515 , even without an ego_vehicle defined, scenarios should still be able to be run with a trigger_point behavior or similar if we want to spawn our own ego vehicle at own location with the correct tags(hero, etc).

Expected behavior
Expected to be able to run without an ego_vehicle defined or at least spawned.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Ubuntu 20.04
• CARLA Version 0.9.15
• Python version 3.7
• Version 0.9.15

[starrye]

Is there any error message?