refactor(): prepare deno std for noUncheckedIndexedAccess

[syhol]

This is part of the effort to rollout the noUncheckedIndexedAccess config for deno std denoland/std#4040

The function getGitHubRepository currently has the implied type

() => { owner: string | undefined, repo: string  | undefined }

It feels cleaner to solve the undefined checking here rather than downstream in deno std _tools/, however if this change isn't acceptable then we can wrap the function in deno std.

Tagging @dsherret since you seem to be a primary maintainer of this repo.

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