image_view: sleep if no new image (backport #1082) (#1083)

Possible fix for #1060<hr>This is an automatic backport of pull request
#1082 done by [Mergify](https://mergify.com).

Co-authored-by: Michael Ferguson <[email protected]>

Author: mergify[bot]

image_view/src/image_view_node.cpp

@@ -296,6 +296,8 @@ void ImageViewNode::windowThread()
       cv::imshow(window_name_, image->image);
       shown_image_.set(image);
       cv::waitKey(1);
+    } else {
+      rclcpp::sleep_for(std::chrono::milliseconds(20));
     }
   }