Add nil check in ListContainerStats

On Windows, `ds.getContainerStats` could return nil for
stopped containers.

Signed-off-by: Xinfeng Liu <XinfengLiu@icloud.com>

core/stats.go

@@ -232,7 +232,9 @@ func (ds *dockerService) ListContainerStats(
 				return nil
 			}
 			mu.Lock()
-			results = append(results, stats)
+			if stats != nil {
+				results = append(results, stats)
+			}
 			mu.Unlock()
 			return nil
 		})