[HDRP] Improves robustness of detail normal mapping when scale = 0 (and not using surface gradients) (#6980)

* Changed the way normal unpacking AG is performed.

* Updated changelog.

* Went for less drastic changes, to minimize unwanted side effects.

Co-authored-by: sebastienlagarde <sebastien@unity3d.com>

Author: eturquin

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -140,6 +140,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed issue in APV with scenes saved only once when creating them.
 - Fixed probe volume baking not generating any probes on mac.
 - Fix a few UX issues in APV.
+- Fixed issue with detail normals when scale is null (case 1399548).
 
 ## [14.0.0] - 2021-11-17
 

com.unity.render-pipelines.high-definition/Runtime/Material/Lit/LitDataIndividualLayer.hlsl

@@ -125,7 +125,7 @@ float3 ADD_IDX(GetNormalTS)(FragInputs input, LayerTexCoord layerTexCoord, float
         #ifdef SURFACE_GRADIENT
         normalTS += detailNormalTS * detailMask;
         #else
-        normalTS = lerp(normalTS, BlendNormalRNM(normalTS, detailNormalTS), detailMask); // todo: detailMask should lerp the angle of the quaternion rotation, not the normals
+        normalTS = lerp(normalTS, BlendNormalRNM(normalTS, normalize(detailNormalTS)), detailMask); // todo: detailMask should lerp the angle of the quaternion rotation, not the normals
         #endif
     #endif
 #else