[ONNX] Allow None as operator argument (#105040)
Needed by 'aten.index.Tensor', where 'indices' is list of optional
tensors.
Related https://github.com/microsoft/onnxscript/pull/862
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105040
Approved by: https://github.com/titaiwangms, https://github.com/thiagocrepaldi
