Handle None response in async_github_get_hacs_default_file method

[coderabbitai[bot]]

In the context of PR https://github.com/hacs/integration/pull/3728 and the discussion https://github.com/hacs/integration/pull/3728#discussion_r1616296455, there's a need to address the behavior when method receives a None response from GitHub API. The suggestion is to raise an exception in such cases. This issue is to track and implement the necessary changes.