How to reproduce:
1. Create a new 3D project
2. Install the Universal RP package from the Package Manager (Window -> Package Manager)
3. Create a new URP Asset in the Project window (Right-click -> Create -> Rendering -> URP -> Pipeline Asset)
4. Select "UniversalRenderPipelineAsset_Renderer" in the Project window
5. Observe the "Post-processing" property in the Inspector

Expected result: "Post-processing" property is checked by default
Actual result: "Post-processing" property is not checked by default

Reproducible with: 11.0.0 (2021.2.0a2), 12.0.0 (2021.2.0a16)
Could not test with: 7.5.3, 10.5.0 (Post-processing property is not in the Forward Renderer)

Note:
- The issue is not reproducible when creating only a Forward Renderer