add null check for c2 tensor conversion (#41096)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41096
The spark spot model had some issues in tensor conversion, see P134598596. It happens when we convert an undefined c10 tensor to caffe2 tensor.
This diff added a null check.
Test Plan: spark spot model runs without problem
Reviewed By: smessmer
Differential Revision: D22330705
fbshipit-source-id: dfe0f29a48019b6611cad3fd8f2ae49e8db5427e
