Print empty char after updated code exercise

To enforce `clear_output` even when the update is not printing anything
we used `print("", end"")` this seems not to always work so we print an
empty char now.

src/scwidgets/exercise/_widget_code_exercise.py

@@ -727,8 +727,8 @@ def _on_click_update_action(self) -> bool:
 
             # The clear_output command at the beginning of the function waits till
             # something is printed. If nothing is printed, it is not cleared. We
-            # enforce it to be invoked by printing an empty string
-            print("", end="")
+            # enforce it to be invoked by printing an empty char
+            print("\0", end="")
 
         return not (raised_error)