github_repository needs to not fail if repository doesn't exist

[ronmpatel]

Affected Resource(s)
github_repository

For a new repository the following will fail the entire plan/apply. It would be nice if we return null so that we could use that to conditionally check if repository exist or not.

Our use case is to create the repository only if does not exist.

Terraform Configuration Files
data "github_repository" "existing_repository" {
  full_name   = "${var.github_org}/${var.repo}"
}

Error Output
GET https://api.github.com/repos// 404 Not Found []

Reference
We have this for branch here