Fix: Handle sparse quadtrees in Fractal Engine (NoneType error)

logos/fractal_engine.py

@@ -356,6 +356,9 @@ class LogosFractalEngine:
     
     def _render_node(self, node, image, x, y, w, h):
         """Recursively render quadtree node"""
+        if node is None:
+            return
+
         if node.is_leaf():
             # Leaf node: render based on matrix_key and heat_state
             if node.matrix_key is not None: