Fix conversion of Transpose operator with missing perm attr

A missing `perm` attr signals that the input dimensions should be reversed.  The
converter translated a missing `perm` attr into an empty permuation list instead
of `None`, causing the operator to fail with an "Invalid permutation" error.

Tested with the `decoder_model.onnx` model from
https://huggingface.co/Mozilla/distilvit/tree/main/onnx.

rten-convert/rten_convert/converter.py

@@ -921,7 +921,7 @@ def op_node_from_onnx_operator(
 
         case "Transpose":
             attrs = sg.TransposeAttrsT()
-            attrs.perm = op_reader.get_attr("perm", "ints", [])
+            attrs.perm = op_reader.get_attr("perm", "ints", None)
 
         case "Trilu":
             attrs = sg.TriluAttrsT()