[Schema Inaccuracy] Pull Request Review Comment `position` should be nullable

[bigfootjon]

Schema Inaccuracy

For each Pull Request Comment in the API, it has a position field attached. Some comments are not attached to a position so this field is null. The Schema does not reflect this nuance.

Expected

position in Pull Request review comments should be marked as nullable

Reproduction Steps

We have observed this in 2 different APIs, either listing PR review comments or fetching a specific comment:

$ curl https://api.github.com/repos/pytorch/tutorials/pulls/2120/comments
$ curl https://api.github.com/repos/pytorch/tutorials/pulls/comments/1024558033

In both cases, the position field is null:

...
"position": null,
...