Remove reference to normal map OS in normal map slot of a material (avoid normal map conversion popup)

Tests/GraphicsTests/RenderPipeline/HDRenderPipeline/Scenes/1xxx_Materials/1201_Lit_Features/Lit_NormalMap_ObjectSpace.mat

         m_Scale: {x: 1, y: 1}
         m_Offset: {x: 0, y: 0}
     - _NormalMap:
-        m_Texture: {fileID: 2800000, guid: 72c21bad8ebd4a043882b302f9837ae3, type: 3}
+        m_Texture: {fileID: 0}
         m_Scale: {x: 1, y: 1}
         m_Offset: {x: 0, y: 0}
     - _NormalMapOS: