It is minor, but if you look at the code for TextInputControl, it is clear that the JavaDoc says empty prompt texts are null, while the code actually implements an empty String. I think the code is fine, but the JavaDoc needs to be updated.

    /**
     * The prompt text to display in the {@code TextInputControl}, or
     * <tt>null</tt> if no prompt text is displayed.
     * @since JavaFX 2.2
     */
    private StringProperty promptText = new SimpleStringProperty(this, "promptText", "") {