The Custom method relies on a theme template of some sort. Are you using a theme php template, as the logic would look something like this to do as you want:
global $searchandfilter;
$sf_current_query = $searchandfilter->get(1024)->current_query();
if ((!$sf_current_query->is_filtered())&&($sf_current_query->get_search_term()=="")) {
echo '<div>Nothing to see here folks!</div>';
} else {
// the theme template PHP code here
}
Change the number 1024 with your form’s ID number (from the shortcode).
This code will likely need to be used within a part the template file that actually outputs the results (around the if query … have … posts bit).