Add nothing to undo/redo

https://forums.ankiweb.net/t/translation-of-nothing-to-undo/31735

ftl/core/actions.ftl

@@ -68,3 +68,5 @@ actions-flag-pink = Pink
 actions-flag-turquoise = Turquoise
 actions-flag-purple = Purple
 actions-set-flag = Set Flag
+actions-nothing-to-undo = Nothing to undo
+actions-nothing-to-redo = Nothing to redo

qt/aqt/operations/collection.py

@@ -60,7 +60,7 @@ def _legacy_undo(*, parent: QWidget) -> None:
 
     if result is None:
         # should not happen
-        showInfo("nothing to undo", parent=parent)
+        showInfo(tr.actions_nothing_to_undo(), parent=parent)
         mw.update_undo_actions()
         return