Fix Invalid Texture Handle Issue in Shadow Atlas

This change fixes an issue where the shadow atlas could end up with an
invalid texture handle after the render pipeline asset is modified and
reloaded.

The previous code was invalidating the output texture handle in some
particular cases, but a new handle was recently added to the atlas
code without updating the invalidation logic. This change simply adds
the new handle to the existing invalidation logic.

The new handle was added as part of #6030 and merged into master as:
c83cb31

This fix may need to be backported to other branches.

com.unity.render-pipelines.high-definition/Runtime/Lighting/Shadow/HDShadowAtlas.cs

@@ -184,6 +184,7 @@ public void InvalidateOutputIfNeeded()
             if (!m_UseSharedTexture)
             {
                 m_Output = TextureHandle.nullHandle;
+                m_ShadowMapOutput = TextureHandle.nullHandle;
             }
         }
 
@@ -233,6 +234,7 @@ internal void CleanupRenderGraphOutput(RenderGraph renderGraph)
                 renderGraph.ReleaseSharedTexture(m_Output);
                 m_UseSharedTexture = false;
                 m_Output = TextureHandle.nullHandle;
+                m_ShadowMapOutput = TextureHandle.nullHandle;
             }
         }