populate empty resonse for IndexStatsResponse and VolumeResponse (#11209

)

Fixes a panic when the underlying response struct is accessed but not
populated after an out-of-bounds request is short-circuited via
`NewEmptyResponse`
[here](https://github.com/grafana/loki/blob/main/pkg/querier/queryrange/limits.go#L155-L164).
This PR ensures the embedded types are not nil in accordance with the
other variants in this function.

(cherry picked from commit 265018b)

pkg/querier/queryrange/codec.go

@@ -1607,9 +1607,13 @@ func NewEmptyResponse(r queryrangebase.Request) (queryrangebase.Response, error)
 			},
 		}, nil
 	case *logproto.IndexStatsRequest:
-		return &IndexStatsResponse{}, nil
+		return &IndexStatsResponse{
+			Response: &logproto.IndexStatsResponse{},
+		}, nil
 	case *logproto.VolumeRequest:
-		return &VolumeResponse{}, nil
+		return &VolumeResponse{
+			Response: &logproto.VolumeResponse{},
+		}, nil
 	default:
 		return nil, fmt.Errorf("unsupported request type %T", req)
 	}