Add default terrain material.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/HDAssetFactory.cs

 
                 newAsset.defaultDiffuseMaterial = Load<Material>(HDRenderPipelinePath + "RenderPipelineResources/DefaultHDMaterial.mat");
                 newAsset.defaultDecalMaterial = Load<Material>(HDRenderPipelinePath + "RenderPipelineResources/DefaultHDDecalMaterial.mat");
+                newAsset.defaultTerrainMaterial = Load<Material>(HDRenderPipelinePath + "RenderPipelineResources/DefaultHDTerrainMaterial.mat");
                 newAsset.defaultShader = Load<Shader>(HDRenderPipelinePath + "Material/Lit/Lit.shader");
 
                 newAsset.debugFontTexture = Load<Texture2D>(HDRenderPipelinePath + "RenderPipelineResources/DebugFont.tga");

ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDRenderPipelineAsset.cs

 
         public override Material GetDefaultTerrainMaterial()
         {
-            return null;
+            return m_RenderPipelineResources.defaultTerrainMaterial;
         }
 
         public override Material GetDefaultUIMaterial()

ScriptableRenderPipeline/HDRenderPipeline/HDRP/RenderPipelineResources/HDRenderPipelineResources.asset

     type: 2}
   defaultDecalMaterial: {fileID: 2100000, guid: 500e733574922d04ea961553b1b26a63,
     type: 2}
+  defaultTerrainMaterial: {fileID: 2100000, guid: 9579db5938031654fbdfe8d222ca0465,
+    type: 2}
   defaultShader: {fileID: 4800000, guid: 6e4ae4064600d784cac1e41a9e6f2e59, type: 3}
   debugFontTexture: {fileID: 2800000, guid: a3ad2df0e49aaa341a3b3a80f93b3f66, type: 3}
   debugDisplayLatlongShader: {fileID: 4800000, guid: c1d1d149a043a5349ba367da6c2051ba,

ScriptableRenderPipeline/HDRenderPipeline/HDRP/RenderPipelineResources/RenderPipelineResources.cs

         // Default Material / Shader
         public Material defaultDiffuseMaterial;
         public Material defaultDecalMaterial;
+        public Material defaultTerrainMaterial;
         public Shader defaultShader;
 
         // Debug

ScriptableRenderPipeline/HDRenderPipeline/HDRP/RenderPipelineResources/DefaultHDTerrainMaterial.mat

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ScriptableRenderPipeline/HDRenderPipeline/HDRP/RenderPipelineResources/DefaultHDTerrainMaterial.mat.meta

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