Unfurl issue comments

[dennissivia]

Unfurling issue comments causes errors

Summary

Sharing a link to a GitHub issue comment leads to an error message.

Visibility

For public repos no message is shown
For private repos an error message is shown, asking the user to contact GH support.

Expected behavior

The GH issue comment is unfurled

Steps to reproduce

• Share an issue comment link on GitHub for a private or public channel
• Link is not unfurled
• For private repositories and error is shown to the user.

Example link:

• Tracking issue for const fn pointers rust-lang/rust#63997 (comment)

Cause

8:56:48.513Z ERROR http: Empty value for parameter 'comment_id': undefined (id=86b1ba8d-141f-40e9-9eef-ebc87e5f0be3)
  HttpError: Empty value for parameter 'comment_id': undefined
      at values.forEach (/Users/.../slack/node_modules/@octokit/rest/plugins/validate/validate.js:66:15)
      at Array.forEach (<anonymous>)
      at Object.keys.forEach.parameterName (/Users/.../slack/node_modules/@octokit/rest/plugins/validate/validate.js:38:12)
      at Array.forEach (<anonymous>)
      at validate (/Users/...s/slack/node_modules/@octokit/rest/plugins/validate/validate.js:15:23)
      at process._tickCallback (internal/process/next_tick.js:68:7)