How to reproduce:
1. Open the "ShaderRepro.zip"
2. Open the "temp" scene
3. Notice, that the nebula fogs are not visible

Actual result: The custom shader fallbacks to diffuse, and the nebula fog is not visible.
Expected result: The custom shader shouldn't fallback and the nebula fog should be visible.

Reproducible with: 5.6.4p4, 2017.1.2p4, 2017.2.1f1, 2017.3.0f2, 2018.1.0a7.

Notes:
If the Directional light has "Shadow Type" set to No shadows, the shader doesn't fallback.
Commenting " Fallback "Diffuse" " (line 59 at Su_StaticStars_Noice shader) makes the shader work correctly, even if the shadows are set on the Directional Light.