fix(github): prevent null pointer dereferencing when using AllowMergeableBypassApply with no required checks on branch protection

[arturhoo]

what

Handle the case where Atlantis is using the feature flag AllowMergeableBypassApply, but a repository does not have a required check under its branch protection rules (it might have, for example, a requirement for at least one approval).

Currently, the code assumes that if a response was returned from https://docs.github.com/en/rest/branches/branch-protection?apiVersion=2022-11-28#get-branch-protection, then a required check will always be defined, which is not the case, leading to the panic seen in the linked issues:

runtime error: invalid memory address or nil pointer dereference
runtime/panic.go:260 (0x44e11c)
runtime/signal_unix.go:837 (0x44e0ec)
github.com/runatlantis/atlantis/server/events/vcs/github_client.go:438 (0xd91157)
github.com/runatlantis/atlantis/server/events/vcs/github_client.go:507 (0xd91717)
github.com/runatlantis/atlantis/server/events/vcs/instrumented_client.go:206 (0xd98beb)
github.com/runatlantis/atlantis/server/events/vcs/proxy.go:80 (0xd9a984)
github.com/runatlantis/atlantis/server/events/vcs/pull_status_fetcher.go:32 (0xd9b255)
github.com/runatlantis/atlantis/server/events/apply_command_runner.go:105 (0xf92cb4)
github.com/runatlantis/atlantis/server/events/command_runner.go:298 (0xf97f43)
runtime/asm_amd64.s:1598 (0x46b2e0)

why

See issues #2663, #3563

tests

I have implemented a new test case that covers the case where:

• Check suite has runs
• Check runs contains runs that have completed with success
• But none of the checks in the suite is a required check

Happy to add more cases if deemed necessary.

references

closes #2663
closes #3563

[jamengual]

LGTM thanks @arturhoo for the contribution