Handle when issue.body is `null`

[rob-b]

Handle parsing the case where issue.body is null. Make the field be optional but default to an empty string

Issue reference:

Submission Checklist:

• [x] Have you followed the guidelines in our Contributing document (for example, is your tree a clean merge)?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Does your submission build?
• [x] Does your submission pass tests?
• [ ] Have you run lints on your code locally prior to submission?
• [ ] Have you updated all of the cabal/nix infrastructure?
• [ ] Is this a breaking change? Have you discussed this?

[rob-b]

This is pretty much the same issue as my previous pr - there's a text field that github doesn't document can be null

[kvanbere]

Thanks, looks good. This is non breaking so I’m happy to merge this right away.

[kvanbere]

Are you fine if I release this soon? Do you see any other changes on the horizon?

[rob-b]

I've not encountered anything else that is likely to require any changes so please do go ahead and make a new release. Thanks for the quick turnaround