Thanks for using the plugin. If I understand your question correctly, you'd like widget to have no text output if it doesn't find any posts. If that is your question, it is possible. You'll want to create a custom HTML template first. Then in your custom HTML template, you want to remove the following lines of code:
<?php else: // We have no posts ?>
<div class="dpe-flexible-posts no-posts">
<p><?php _e( 'No post found', 'flexible-posts-widget' ); ?></p>
By removing the code in the PHP
else block, the widget will display nothing if there are no results.
By the way, it appears the site you referenced is written in Italian. If you created an Italian translation for Flexible Posts Widget, I would love to include it with the plugin. Would you be open to sharing your translation files (PO & MO) with me?