Do not return a request in buildRequests on error

In some cases, the buildCdbRequest function might return a falsy value,
in case of error in creating the request or if we know in advance that
this request will return a no-bid.

In this case, the buildRequests() method should not return a request,
causing a no-bid.

modules/criteoBidAdapter.js

@@ -63,7 +63,9 @@ export const spec = {
       data = buildCdbRequest(context, bidRequests);
     }
 
-    return { method: 'POST', url, data, bidRequests };
+    if (data) {
+      return { method: 'POST', url, data, bidRequests };
+    }
   },
 
   /**