[torchlib] Explicitly check for `None` in sequence inputs

Potentially. As proposed in

              Should we consider elif None? It's easier to catch what we are not expecting.

_Originally posted by @titaiwangms in https://github.com/microsoft/onnxscript/pull/883#discussion_r1265762287_
            