If search form empty=> no result

You can change that by going to Events > Settings > Formatting > Events and editing the No events message field.

Sorry, I misunderstood your question. That’s not an out-of-the-box option, you’d need to copy and modify the /events-manager/templates/templates/events-search.php template.

Take a look at this tutorial that shows you how to make upgrade safe edits to Events Manager templates:
http://wp-events-plugin.com/documentation/using-template-files/