Fix optional types in CheckSuite struct

[maflcko]

A few types must be optional, according to the docs and to avoid runtime errors.

Docs: https://docs.github.com/en/rest/checks/suites?apiVersion=2022-11-28#list-check-suites-for-a-git-reference (They are optional in the schema)

I used for testing:

    octocrab::instance()
        .checks("llvm", "llvm-project")
        .list_check_suites_for_git_ref(octocrab::params::repos::Commitish(
            "f749d0dee693c6082227862757cf3ce47568ab4b".to_string(),
        ))
        .per_page(2)
        .send()
        .await
        .unwrap();

Previously, it crashed. Now, it passes.

[XAMPPRocky]

Thank you for your PR!