* Fix datepickers for sorting submissions

An empty date format is passed to the jQuery datepickers on the submissions page if no date format setting has been saved in the database. This breaks the datepicker (an empty date string is passed back to the input field).

This commit introduces a datepicker_args object which can be safely passed to the datepicker whether or not any date format has been set.

No changes were made to the minified assets as I don't know what settings you're using for minification.

I didn't want to go beyond the simple bug fix. However, I think a more extensible approach would be to pass a filtered datepicker_args array to the script in [/includes/admin/scripts.php](https://github.com/wpninjas/ninja-forms/blob/develop/includes/admin/scripts.php#L44). Something like this:

```
wp_localize_script( 'ninja-forms-admin', 'ninja_forms_settings', array( 'datepicker_args' => apply_filters( 'admin_submissions_datepicker_args', array( 'dateFormat' => $date_format) ) ) );
```

Just a suggestion.