Return empty string rather than None because Mako functions write by default; add comments as well.

templates/refresh_frames.mako

@@ -1,9 +1,11 @@
 ## Include JavaScript code to refresh Galaxy application frames as needed.
 <%def name="handle_refresh_frames()">
+    ## If no refresh frames, print nothing.
     <%
-        if not refresh_frames: return
+        if not refresh_frames: return ''
     %>
 
+    ## Write JavaScript to refresh specified frames.
     <script type="text/javascript">
     %if 'everything' in refresh_frames:
         parent.location.href="${h.url_for( controller='root' )}";