Support deleted PR branch

[thehabbos007]

When a PR branch is deleted the repository.pullRequest.headRefs is null. This change is aware of that.
Currently the following error happens if the action is used on a PR with a deleted branch (closed or merged):

Error: Cannot read property 'name' of null
(node:1474) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'name' of null
    at pullRequestDetails (/home/runner/work/_actions/storyai/pull-request-comment-branch/v1/webpack:/pull-request-comment-branch/src/PullRequests.ts:38:1)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at run (/home/runner/work/_actions/storyai/pull-request-comment-branch/v1/webpack:/pull-request-comment-branch/src/main.ts:9:1)

It is understandable if it is not desirable to have null refs in steps, though :)