USDScene : Ignore attributes without authored values

This prevents us from having to later return a null attribute from `readAttribute()`, which in turn becomes a warning in Gaffer.

Author: johnhaddon

contrib/IECoreUSD/src/IECoreUSD/USDScene.cpp

@@ -762,9 +762,9 @@ bool USDScene::hasAttribute( const SceneInterface::Name &name ) const
 		pxr::TfToken kind;
 		return model.GetKind( &kind );
 	}
-	else if( AttributeAlgo::findUSDAttribute( m_location->prim, name.string() ) )
+	else if( auto attribute = AttributeAlgo::findUSDAttribute( m_location->prim, name.string() ) )
 	{
-		return true;
+		return attribute.HasAuthoredValue();
 	}
 	else
 	{
@@ -813,6 +813,10 @@ void USDScene::attributeNames( SceneInterface::NameList &attrs ) const
 	std::vector<pxr::UsdAttribute> attributes = m_location->prim.GetAuthoredAttributes();
 	for( const auto &attribute : attributes )
 	{
+		if( !attribute.HasAuthoredValue() )
+		{
+			continue;
+		}
 		IECore::InternedString name = IECoreUSD::AttributeAlgo::cortexAttributeName( attribute );
 		if( name.string().size() )
 		{

contrib/IECoreUSD/test/IECoreUSD/USDSceneTest.py

@@ -2388,6 +2388,7 @@ def assertExpectedAttributes( sphere ) :
 				"namespaced:test",
 				"nonexistent",
 				"radius",
+				"test:noAuthoredValue",
 			] :
 				self.assertFalse( sphere.hasAttribute( name ) )
 				self.assertIsNone( sphere.readAttribute( name, 0 ) )

contrib/IECoreUSD/test/IECoreUSD/data/customAttribute.usda

@@ -23,6 +23,7 @@ def Sphere "sphere"
 	)
 	float primvars:arnold:disp_height = 0.5
 	int primvars:arnold:poly_mesh:subdiv_iterations = 3
+	custom string[] test:noAuthoredValue
 }
 def Xform "a"
 {