A PR with absolutely no body causes errors in hubcaps

[grahamc]

NixOS/nixpkgs#34372 had no body, and hubcaps couldn't handle this, and would return an Err when trying to parse the issue.

[grahamc]

This came up again:

{
  "url": "https://api.github.com/repos/NixOS/nixpkgs/issues/34454",
  "repository_url": "https://api.github.com/repos/NixOS/nixpkgs",
  "labels_url": "https://api.github.com/repos/NixOS/nixpkgs/issues/34454/labels{/name}",
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  "events_url": "https://api.github.com/repos/NixOS/nixpkgs/issues/34454/events",
  "html_url": "https://github.com/NixOS/nixpkgs/pull/34454",
  "id": 293150228,
  "number": 34454,
  "title": "cups-filters: 1.16.0 -> 1.20.0",
  "user": {
    "login": "layus",
    "id": 632767,
    "avatar_url": "https://avatars3.githubusercontent.com/u/632767?v=4",
    "gravatar_id": "",
    "url": "https://api.github.com/users/layus",
    "html_url": "https://github.com/layus",
    "followers_url": "https://api.github.com/users/layus/followers",
    "following_url": "https://api.github.com/users/layus/following{/other_user}",
    "gists_url": "https://api.github.com/users/layus/gists{/gist_id}",
    "starred_url": "https://api.github.com/users/layus/starred{/owner}{/repo}",
    "subscriptions_url": "https://api.github.com/users/layus/subscriptions",
    "organizations_url": "https://api.github.com/users/layus/orgs",
    "repos_url": "https://api.github.com/users/layus/repos",
    "events_url": "https://api.github.com/users/layus/events{/privacy}",
    "received_events_url": "https://api.github.com/users/layus/received_events",
    "type": "User",
    "site_admin": false
  },
  "labels": [

  ],
  "state": "open",
  "locked": false,
  "assignee": null,
  "assignees": [

  ],
  "milestone": null,
  "comments": 3,
  "created_at": "2018-01-31T13:03:25Z",
  "updated_at": "2018-02-02T02:23:59Z",
  "closed_at": null,
  "author_association": "CONTRIBUTOR",
  "pull_request": {
    "url": "https://api.github.com/repos/NixOS/nixpkgs/pulls/34454",
    "html_url": "https://github.com/NixOS/nixpkgs/pull/34454",
    "diff_url": "https://github.com/NixOS/nixpkgs/pull/34454.diff",
    "patch_url": "https://github.com/NixOS/nixpkgs/pull/34454.patch"
  },
  "body": null,
  "closed_by": null
}

[grahamc]

I sent a PR upstream softprops/hubcaps#107