By default, there is a blank option in a Dynamic field dropdown. You can change the label on that option with the code below.

Use this format if you would like to add an error to the form.

If you would like to allow your users to edit their entry for a limited amount of time, you can add this to a child theme's functions.php file or the 'Code Snippets' plugin. Just replace 19 with the ID of your form and replace 24 with the number of hours that you want to allow editing.