The API currently defaults to v3, which uses usernames instead a channel id, so when you’re calling the streams endpoint with an id the API is looking for a channel where the username matches that id, which it doesn’t and so returns null streams. If you add api_version=5 to the querystring, or Accept: application/vnd.twitchtv.v5+json as a header in the request it should give you the response you are looking for.
Also, to get the id of a channel in the first place there is a page in the docs https://dev.twitch.tv/docs/v5/guides/using-the-twitch-api#translating-from-user-names-to-user-ids that explains how to translate a username to ID with the v5 API.