Camera preview shows only Skybox with forward rendering path and perspective projection or ortographic projection and both forward and deferred rendering paths.
Happens when there a skybox is set to the material in Scene Lighting settings.

Reproduction steps:

1. Open the project
2. Select Main Camera game object
3. Select Forward Rendering Path
4. Check preview screen in scene view
5. Check what's on game view

Expectation: same view
What happens: in preview window, only skybox is being shown

Reproduced on: 5.5.0b8, 5.5.0f1, 5.5.2f1, 5.6.0a1, 5.6.0b9, 5.6.0b10, 2017.1.0a2
Regression on: 5.5.0b8.