consistent v2 weight reader behavior

Author: calad0i

hls4ml/converters/keras_v3_to_hls.py

@@ -168,7 +168,7 @@ def get_weights_data(self, layer_name, var_name):
                 for w in layer.weights:
                     if var_name in w.name:
                         return np.array(w)
-                raise ValueError(f"Variable {var_name} not found in layer {layer.name}")
+                return None
 
         reader = DummyReader()
         input_shapes = [list(t.shape) for t in inp_tensors]