fix: fix nullpointerexception when body is null

[kvanzuijlen]

An exception will occur when the body is null (empty) and a pr description check is configured. This change will fall back to an empty string when the body is null (and thus empty).

Piece of webhook content:

{
  "action": "reopened",
  "number": 1144,
  "pull_request": {
    ...,
    "body": null,
    ...
  },
  ...
}

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This change is