[Schema Inaccuracy] pull-request.head.repo is not required

[jamesmbourne]

Schema Inaccuracy

If a pull request is created against a fork and the fork deleted, it is possible for pull-request.head.repo to be null.

Expected

The pull-request.head.repo property is non-required

Reproduction Steps

$ curl -X GET https://api.github.com/repos/{repo}/{repoName}/pulls/2

[mattpage]

This is fixed. See #424