github: handle more kinds of incomplete events

[jclulow]

We have on occasion received events from GitHub that pretty heavily imply they are not using a strongly typed language in enough of their inner workings. Recently, we received a pull request event where the sender field was entirely absent, and the head of the pull request was not completely filled out:

            head: PullRequestCommit {
                label: None,
                ref_: "tracing-appender-localtime",
                sha: "6b77405e825846c4e6e2cc1ca9544287ef54c12c",
                user: None,
                repo: None,
            },

As we don't currently need those fields, we can just make them optional.

[emilyalbini]

The idea of the change looks good to me, but there are a bunch of CI failures on this one :(