Passing a coupon code to a desktop application

Question

I've been trying to create a solution using URL parameters for discount codes. The goal is to be able to share a URL with someone with the discount in the URL, which then gets passed through to app sign up.

I thought I had this all working until I noticed people without a discount were seeing 'null' entered in the discount code box at checkout. So everyone without a discount was receiving an error.

Is there anything clever I can add in to stop the entry of null passing through? Here's the code:

<script>
        // create urlParams variable (constant) from URLSearchParams class using current window
           const urlParams = new URLSearchParams(window.location.search);
        // set UTM medium, source and campaign variables (constants) based on results of URSearchParams
           const coupon = urlParams.get('coupon')
        // get the RSVP button element
           var couponLinks = document.getElementsByClassName("button");
        // edit RSVP button element property by appending the URL parameters
           for (let couponLink of couponLinks){
               couponLink.href += "?" + "coupon=" + coupon;
           }
</script>
<script>
        // create urlParams variable (constant) from URLSearchParams class using current window
           const urlPass = new URLSearchParams(window.location.search);
        // set UTM medium, source and campaign variables (constants) based on results of URSearchParams
           const param = urlPass.get('coupon')
        // get the RSVP button element
           var programLinks = document.getElementsByClassName("nav-link program");
        // edit RSVP button element property by appending the URL parameters
           for (let programLink of programLinks){
               programLink.href += "?" + "coupon=" + param;
           }
</script>

I can update all CTAs to include 'coupon=' so it doesn't pass over null, but this isn't ideal.