It always has 'pull_request' key. When there is no pull request, all the values inside this hash are nil. Otherwise they contain the URL to that PR
For eg. with no PR

``` 
pull_request: {
html_url: null,
diff_url: null,
patch_url: null
}
```
And with PR

```
pull_request: {
html_url: "https://github.com/codetriage/codetriage/pull/166",
diff_url: "https://github.com/codetriage/codetriage/pull/166.diff",
patch_url: "https://github.com/codetriage/codetriage/pull/166.patch"
}
```