[HDRP][DXR] Skip adding higher level Renderer to RTAS if null

[remi-chapelain]

Purpose of this PR

Fix for this one. (forum thread)

• Few month ago, made this PR to fix an LOD renderer issue with raytracing when building RTAS. LOD wasn't supported back then but we were still looping through higher level LOD to keep a reference so they weren't taken into account when processing the rest of the renderer.
• In the meantime, support for LOD in raytracing was added and now we loop through every LOD and add them to the RTAS.
• The problem is: If your setup is bad and there's missing renderer in your LOD group, the RayTracingManager still tries to add the renderer and fail. Which makes the render fail => black screen.

This PR skips adding the renderer to the RTAS if it's null which can happen in large projects.
The only "bad things" is that the fail is silent and ray traced effects won't be applied on the LOD level if it does not have a renderer.
The Check for RayTracing issue menu item still shows which gameobject has issues if you want to fix it.

Gif shows that now we can have missing renderer and still have a render. The RTAO will show only on the third level lod which is the only one that has a renderer. Still a bad setup but no errors and we keep the rendering working.

Check scene content for raytracing setup issues

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Testing status

• Checked the repro case ✔️
• Checked that the RT effect were not applied if renderer was not filled in one of the LOD Level ✔️
• Launch DXR tests locally ✔️

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Comments to reviewers

N/A

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[TomasKiniulis]

Looks good to me, testing is sufficient.

[anisunity]

That code will soon be gone (removed by this PR #5988) so we shouldn't be changing it