Always fail with PR have no description

[lib/src/Gren.js#L1024](https://github.com/github-tools/github-release-notes/blob/master/lib/src/Gren.js#L1024)
```js
const matches = pr.body.match(re);
```

If PR have no description, this line always fail.

```
Getting all merged pull requestsTypeError: Cannot read property 'match' of null
at _context17.t1 (/home/filepang/.nvm/versions/node/v12.18.2/lib/node_modules/github-release-notes/dist/Gren.js:1760:59)
at Array.map ()
at Gren._callee17$ (/home/filepang/.nvm/versions/node/v12.18.2/lib/node_modules/github-release-notes/dist/Gren.js:1773:77)
at tryCatch (/home/filepang/.nvm/versions/node/v12.18.2/lib/node_modules/github-release-notes/node_modules/regenerator-runtime/runtime.js:62:40)
at Generator.invoke [as _invoke] (/home/filepang/.nvm/versions/node/v12.18.2/lib/node_modules/github-release-notes/node_modules/regenerator-runtime/runtime.js:296:22)
at Generator.prototype. [as next] (/home/filepang/.nvm/versions/node/v12.18.2/lib/node_modules/github-release-notes/node_modules/regenerator-runtime/runtime.js:114:21)
at step (/home/filepang/.nvm/versions/node/v12.18.2/lib/node_modules/github-release-notes/node_modules/babel-runtime/helpers/asyncToGenerator.js:17:30)
at /home/filepang/.nvm/versions/node/v12.18.2/lib/node_modules/github-release-notes/node_modules/babel-runtime/helpers/asyncToGenerator.js:28:13
```

Start at lib/src/Gren.js#L1024 and compare the reported stack trace with the pull-request processing path. Reproduce the failure using a merged pull request whose body is null, then verify that collecting merged pull requests completes without the TypeError and still produces the expected release-note data.

由索引模型根據 Issue 內容生成。