_content/js: fix tour playground

Fixed a malformed request URL when options.backend is undefined.

Fixes golang/go#49936
Fixes golang/tour#1281

Change-Id: I9c2c29d2d67e3bffe0d490517f6a9168a6d4dc46
Reviewed-on: https://go-review.googlesource.com/c/website/+/368914
Trust: Jamal Carvalho <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>
Run-TryBot: Jamal Carvalho <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: jamalc

_content/js/playground.js

@@ -123,7 +123,7 @@ function HTTPTransport(enableVet) {
       seq++;
       var cur = seq;
       var playing;
-      $.ajax('/_/compile?backend='+options.backend, {
+      $.ajax('/_/compile?backend=' + (options.backend || ''), {
         type: 'POST',
         data: { version: 2, body: body, withVet: enableVet },
         dataType: 'json',