Bug when clicked "Re-run" button

[MVrachev]

Describe the bug
This bug appears when you click "Re-run all checks" or "Re-run failed checks button on the checks tab in one repository.

This bug happens when you already have a pull request from a forked repository to an original repository and click the above buttons.
This bug doesn't affect when you create a pull request directly into the repository without a forked repository.

To Reproduce
Steps to reproduce the behavior:

1. Make a pull request from a forked repository to an original repository
2. Click on the buttons showed above
3. You will get the error "Cannot read property 'filter' of undefined" as a result.

[MVrachev]

After investigation, I found out that GitHub doesn't send me a pull request attribute in this specific situation when the pull request is from a forked repository to an original repository and I clicked the buttons "Re-run" I get an empty pull request attribute:

in the other situation when I create a pull request directly without using a forked repository and I clicked the "Re-run buttons" the pull request field is there:

[joshuagl]

Apologies, accidentally triggered the shortcut to close...

[joshuagl]

The GitHub documentation for CheckSuiteEvent says:

Note: The Checks API only looks for pushes in the repository where the check suite or check run were created. Pushes to a branch in a forked repository are not detected and return an empty pull_requests array and a null value for head_branch.

Which feels related to this issue. We know from #199 that we receive the CheckSuiteEvent for both the target repository against which a pull request was filed and a source repository when the app is also installed there. I think we'll need to filter the CheckSuiteEvent's to make sure they are being triggered for the right repository.