Is there no 2xx code that indicates "this server is healthy and could be configured to serve real web pages, so we're telling you it's a sort of success"? Doesn't look like it.
Proposal: 209 Healthy With Minimum Configuration
Proposal: 209 Healthy With Minimum Configuration
Seems to fit that bill well. Though I'm not sure exactly what your browser would show as it's meant not to cause a direct to a new page.
Give no information about what it is.
Give a status to a monitor that it is alive.