Fix nonclearing output when nothing is outputed

When the the update does not print anything the output is not cleared,
since it waits for an output to be cleared. We fix this by printing an
empty string to invoke the clearning of the output

src/scwidgets/exercise/_widget_code_exercise.py

@@ -657,6 +657,11 @@ def _on_click_update_action(self) -> bool:
                 raised_error = True
                 raise e
 
+            # 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='')
+
         return not (raised_error)
 
     def run_update(self):