The compositor was disabled due to a validation error is thrown on enabling Compositor in Empty (Built-in) scene.

Steps to repro:
1. Create a new HDRP project.
2. File > New Scene > Select > Basic (Built-in) scene.
3. Windows > Render Pipeline > Graphics Compositor.
4. Enable Compositor.

Actual Result:
The compositor was disabled due to a validation error is thrown in the console.

Expected Result:
No errors in the console.

Reproducible in:
2020.2.0b8, 2020.2.0a15 with Package Version 10.0.0-preview.27, 9.0.0-preview.54, 9.0.0-preview.33.

Note: Empty(Built-in) scene option is available from 2020.2.0a15.

Environment:
Occurring on Windows 10 & Mac 10.15.

Stacktrace:
The compositor was disabled due to a validation error in the configuration.
UnityEngine.Rendering.HighDefinition.Compositor.CompositionManager:Start () (at library/packagecache/com.unity.render-pipelines.high-definition@10.0.0-preview.27/runtime/compositor/compositionmanager.cs:326)