Return a json response in scheduling_policy handler (#10904)

Return an empty json response in the `scheduling_policy` handler.

This prevents errors of the form:

```
Error: receive body: error decoding response body: EOF while parsing a value at line 1 column 0
```

when setting the scheduling policy via the `storcon_cli`.

part of #9011.

storage_controller/src/http.rs

@@ -1354,10 +1354,7 @@ async fn handle_safekeeper_scheduling_policy(
         .set_safekeeper_scheduling_policy(id, body.scheduling_policy)
         .await?;
 
-    Ok(Response::builder()
-        .status(StatusCode::NO_CONTENT)
-        .body(Body::empty())
-        .unwrap())
+    json_response(StatusCode::OK, ())
 }
 
 /// Common wrapper for request handlers that call into Service and will operate on tenants: they must only