Enhance Flatten layout handling: update image computation to handle N…

…one outputs and add unit test for empty layout scenario.

model_api/python/model_api/visualizer/layout.py

@@ -45,5 +45,7 @@ def __init__(self, *args: Type[Primitive]) -> None:
     def __call__(self, scene: Scene) -> PIL.Image:
         _image: PIL.Image = scene.base.copy()
         for child in self.children:
-            _image = self._compute_on_primitive(child, _image, scene)
+            output = self._compute_on_primitive(child, _image, scene)
+            if output is not None:
+                _image = output
         return _image

tests/python/unit/visualizer/test_layout.py

@@ -19,3 +19,9 @@ def test_flatten_layout(mock_image: Image, mock_scene: Scene):
     expected_image = Image.blend(mock_image, overlay, 0.4)
     mock_scene.layout = Flatten(Overlay)
     assert mock_scene.render() == expected_image
+
+
+def test_flatten_layout_with_no_primitives(mock_image: Image, mock_scene: Scene):
+    """Test if the layout is created correctly."""
+    mock_scene.layout = Flatten()
+    assert mock_scene.render() == mock_image