Inductor cpp wrapper: fix optional tensor input (#106847)
Fix cpp wrapper failure on `clip` in Torchbench:
```
RuntimeError: tensor does not have a device
```
An `optional<at::Tensor>` variable with value equal to `at::Tensor()` will be considered as _contains value_. When it's converted to `bool`, it returns `true`. While for `None` in python, when converting it to `bool`, `false` is returned.
Fix it to be an optional variable that _does not contain a value_.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106847
Approved by: https://github.com/jgong5, https://github.com/jansel
