net/http: Fetch API-based Transport panics when request has no body

[dmitshur]

The req.Body.Close() calls were refactored in CL 458395 to happen in the success/failure callbacks. The original location was inside an if req.Body != nil block, but the new location missed that check, causing Transport.RoundTrip to panic whenever the request doesn't have a body.

CC @golang/js, @golang/wasm.

[gopherbot]

Change https://go.dev/cl/503676 mentions this issue: net/http: close req.Body only when it's non-nil on js