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,
    ...
  },
  ...
}

---

This change is 

[kvanzuijlen]

@KostyaSha can you create a new release?

[KostyaSha]

in the evening