Explicitly check for Array of previous comments

[mxstbr]

Closes #577

[sunshinejr]

Oh, I think I found the underlying issue. previousComments are fetched using the URL:

https://api.github.com/repos/${repo}/pulls/${prID}/comments

And because this is an issue, not a PR, it is trying to get inline comments for non-existing PR:

https://api.github.com/repos/withspectrum/spectrum/pulls/2994/comments

And there is an error response from the GitHub API instead of an empty array.

[mxstbr]

Whaaat! A proper PR would be much appreciated 🙏

[sunshinejr]

Oh, this PR is good I think. The "proper" fix might be to check if this is a PR/issue and use inline comments based on that. I don't know if it's possible to do currently, though.

[orta]

OK, this seems fine for me right now 👍