`GHIssue#isPullRequest` is false despite being a PR

[Haarolean]

Describe the bug
isPullRequest in GHIssue returns false due to pull_request == null condition. The PR object was acquired via payload.getPullRequest() method of GHEventPayload.PullRequest class.

To Reproduce
Steps to reproduce the behavior:

1. Get an instance of GHEventPayload.PullRequest event
2. Get the pr entity via payload.getPullRequest()
3. Observe that despite object being an instance of GHPullRequest the return value of isPullRequest method is false.

Expected behavior
expected true