stream settings: Fix error in updates of stream description.

When removing the description from a stream (i.e. setting it to ""),
the UI was not correctly updating the description.  This is because we
were checking incorrectly for a falsey value, rather than the specific
value undefined (which means the description wasn't changed).

static/js/stream_data.js

@@ -159,7 +159,7 @@ exports.get_subscriber_count = function (stream_name) {
 };
 
 exports.render_stream_description = function (sub) {
-    if (sub.description) {
+    if (sub.description !== undefined) {
         sub.rendered_description = marked(sub.description).replace('<p>', '').replace('</p>', '');
     }
 };