Empty JSON responses handled gracefully

The HTTP Request node was throwing "Invalid JSON in response body" errors when APIs returned successful responses with empty bodies, even though the content-type header specified application/json and content-length was 0. This caused workflows to fail on valid API responses.

Now the node handles empty JSON responses gracefully by returning an empty object {} instead of attempting to parse an empty string. The fix applies to both autodetect and explicit JSON response format modes, ensuring consistent behavior regardless of how users configure the node.

This matters because many REST APIs return empty bodies on successful operations like POST requests that create resources, status 201 responses with no body, or DELETE operations that return no content. Workflows that integrate with these APIs should not fail due to empty but valid responses.

The change is isolated to the HTTP Request node implementation in the nodes-base package.