Fix controlnet bug.

comfy/controlnet.py

@@ -143,7 +143,7 @@ class ControlNet(ControlBase):
                 if control_prev is not None:
                     return control_prev
                 else:
-                    return {}
+                    return None
 
         output_dtype = x_noisy.dtype
         if self.cond_hint is None or x_noisy.shape[2] * 8 != self.cond_hint.shape[2] or x_noisy.shape[3] * 8 != self.cond_hint.shape[3]: