godoc/static: fix handling of playground snippets with no output

Fixes golang/go#11903

Change-Id: Ia7f85d83f1f697fd787a87cd74971f6cf0540792
Reviewed-on: https://go-review.googlesource.com/12751
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

godoc/static/playground.js

@@ -49,7 +49,7 @@ function HTTPTransport() {
 		var timeout;
 		output({Kind: 'start'});
 		function next() {
-			if (events.length === 0) {
+			if (!events || events.length === 0) {
 				output({Kind: 'end'});
 				return;
 			}