I know that it's just a tiny graphic problem but do you also get a white line (or extra padding) after the last entry in the context menu?
It's quite possibly a bug. Using Ubuntu 19.04, I see the extra space in one context menu but not in another:
Extra space at the bottom of this context menu
No extra space at the bottom of this context menu
If you want to file a bug, there are things you could do first to make sure the developers' task is made easier.
Verify if the issue exists in another distribution you know uses the latest vanilla GNOME such as Fedora 30. If the issue appears there as well, you'd be better off approaching upstream and not the Ubuntu team here.
See if the problem goes away using the built-in Adwaita theme. In this specific case, I've used Adwaita-Dark and so it doesn't appear to be a theming issue caused by using a "non-standard" theme.
If you see the issue only when not using Adwaita, you should get in touch with the developer of the concerned theme. The reason for doing so is detailed in Please don’t theme our apps.
This bug seems to be triggered by the "paste into folder" feature.
If you copy a file/folder to your clipboard, then there's no empty space.