Checkbox上的JavaScript警报出错

I have a problem in my JavaScript code.

I have a HTML form in which I use table values that are fetched from Mysql table. I use an array to print my all values in my table. Here is output of my form:

After that I use JavaScript to alert: "Please select at least one opinion (Mutual Fund)."

Here is my code

    function checkRatings(){
    var checked = [].reduce.call(document.forms[0].opinion, function(current, item) {
    if (item.checked) {
      current.push(item.value);
    }
    return current;
  }, []);
  if (checked=='')
    {
        alert("Please select at least one opinion");
        return;
    }
 else 
    document.getElementById("form_id").submit();}

This code working fine when I have multiple opinions (Mutual Fund) just like as above figure, but it's not working when I have only single opinion. As on this image:

In this scenario after I check opinion it gives me the alert again and again and could not proceed it...

Can anyone please help me?