github_repository_collaborator error on read when repository 404

[paultyng]

When a repository is deleted manually, a read for a collaborator resource on that repository errors instead of just removing from state on a 404:

Error: GET https://api.github.com/repos/terraform-providers/terraform-provider-atlas/invitations?per_page=100: 404 Not Found []

If the repo is 404, this should exit gracefully and just set ID to empty string, not propagate the error:

https://github.com/terraform-providers/terraform-provider-github/blob/70683693d1a602276f73ce687b1faec9e3607416/github/resource_github_repository_collaborator.go#L100

here is how it is done on the repo resource itself:

https://github.com/terraform-providers/terraform-provider-github/blob/70683693d1a602276f73ce687b1faec9e3607416/github/resource_github_repository.go#L276-L280