Fixed default materials for UI/Line etc.

Assets/ScriptableRenderPipeline/HDRenderPipeline/HDRenderPipeline.cs

 
         public override Material GetDefaultParticleMaterial()
         {
-            return m_DefaultDiffuseMaterial;
+            return null;
-            return m_DefaultDiffuseMaterial;
+            return null;
-            return m_DefaultDiffuseMaterial;
+            return null;
-            return m_DefaultDiffuseMaterial;
+            return null;
-            return m_DefaultDiffuseMaterial;
+            return null;
-            return m_DefaultDiffuseMaterial;
+            return null;
-            return m_DefaultDiffuseMaterial;
+            return null;
         }
 
         public void ApplyDebugDisplaySettings()