Fix a crash when there is no apdex data.

This can happen when all hits are detected as "no_module" for ERP5 stats.

apachedex/__init__.py

@@ -825,10 +825,12 @@ def asHTML(out, encoding, per_site, args, default_site, period_parameter_dict,
     if site_id is None:
       site_id = default_site
     out.write('<h1>Site: %s</h1>' % unquoteToHtml(site_id, encoding))
+    apdex_data = data.getApdexData()
+    if apdex_data:
       out.write(
         graphPair(
           prepareDataForGraph(
-          data.getApdexData(),
+            apdex_data,
             date_format,
             placeholder_delta,
           ),