For those wo are looking for a more complete solution, add this in your (child theme’s) functions.php file:

add_filter('avf_logo_final_output', 'avia_remove_default_logo', 10, 6);
function avia_remove_default_logo($logo, $use_image, $headline_type, $sub, $alt, $link){
	if(strpos($logo,'enfold/images/layout/logo.png'))
		return '';
	else
		return $logo;
}

Variable $use_image is always the path to the default logo, so you can’t use that.

This generates no output when no logo is uploaded.