Fix bug when sess.run returns list of empty list

[MatchaChoco010]

I have this error when run onnxsim.

Traceback (most recent call last):
  File "F:\github.com\MatchaChoco010\streaming-vc-test\streaming_asr\extract_onnx.py", line 59, in <module>
    enc, check = simplify(enc)
  File "F:\github.com\MatchaChoco010\streaming-vc-test\.venv\lib\site-packages\onnxsim\onnx_simplifier.py", line 186, in simplify
    model_opt_bytes = C.simplify(
  File "F:\github.com\MatchaChoco010\streaming-vc-test\.venv\lib\site-packages\onnxsim\onnx_simplifier.py", line 248, in Run
    return [
  File "F:\github.com\MatchaChoco010\streaming-vc-test\.venv\lib\site-packages\onnxsim\onnx_simplifier.py", line 249, in <listcomp>
    onnx.numpy_helper.from_array(x).SerializeToString() for x in output_arrs
  File "F:\github.com\MatchaChoco010\streaming-vc-test\.venv\lib\site-packages\onnx\numpy_helper.py", line 114, in from_array
    tensor.dims.extend(arr.shape)
AttributeError: 'list' object has no attribute 'shape'

This error seems to occur when sess.run returns a list of an empty list instead of a list of numpy arrays. When sess.run returns a list, I replaced it with an empty numpy array and the error disappeared.