Show text without alert when no checkbox is cecked

I have this js, but is working just when i delete de "nothing" field, but is ugly in my site if not showme couse i have no checkbox cheked.

js:

setInterval(function(){
   section[0].innerHTML = '';
   for(var i = 0; i < checkbox.length; i++){
     if(checkbox[i].checked){
     section[0].innerHTML += checkbox[i].value;        
     } else {
     section[0].innerHTML = 'nothing';
     }
   }
 }, 10);

html

Checked checkbox: <b><section></section></b>

This js is woking fine, but i wanna add nothing field:

setInterval(function(){
   section[0].innerHTML = '';
   for(var i = 0; i < checkbox.length; i++){
     if(checkbox[i].checked){
     section[0].innerHTML += checkbox[i].value;        
     }
   }
 }, 10);