ComboBox must never hard-code the displayValue if it has a StringConverter - it must pass all values (including null, aka: empty selection) to the converter and display the value it receives.

There's a bit lee-way, if it has both a converter and a prompt ... then probably the prompt should win as it is specialized for the empty-selection case.

    /**
     * Converter must be used always (if available)
     *
     * Run:
     * - expected: display value "none"
     * - actual: empty
     */
    public class ComboBoxStringConverter extends Application
    {
      public static void main(String[] args)
      {
        Application.launch(args);
      }
    
      @Override
      public void start(Stage stage) throws Exception
      {
        final ComboBox<Integer> cb = new ComboBox<Integer>();
        cb.getItems().setAll(1, 2, 3);
        StringConverter converter = new StringConverter() {
            @Override public String toString(Object arg0) {
                if (arg0 == null) return "none";
                return "item " + arg0; }
            @Override public Integer fromString(String arg0) { throw new UnsupportedOperationException(); }
        };
        cb.setConverter(converter);
        VBox c = new VBox(cb);
        stage.setScene(new Scene(c, 500, 400));
        stage.setTitle(System.getProperty("java.version"));
        stage.show();
      }
    }