Fixed the fallback sun for volumetric clouds having a non null intensity (case 1353955).

[anisunity]

https://fogbugz.unity3d.com/f/cases/1353955/
The volumetric clouds require a sun to work, previously a fallback sun was created, vertical and had an intensity of 1 (pre-exposed). Users prefer to have a black sun, let's do that.

Testing status
Checked that it works, trivial enough

[github-actions]

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HDRP
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[remi-chapelain]

Change makes sense and is more inline with what users expect when there's no sun. ✔️