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

@@ -825,17 +825,19 @@ 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))
-    out.write(
-      graphPair(
-        prepareDataForGraph(
-          data.getApdexData(),
+    apdex_data = data.getApdexData()
+    if apdex_data:
+      out.write(
+        graphPair(
+          prepareDataForGraph(
+            apdex_data,
+            date_format,
+            placeholder_delta,
+          ),
           date_format,
-          placeholder_delta,
-        ),
-        date_format,
-        graph_period,
+          graph_period,
+        )
       )
-    )
     out.write(data.asHTML(date_format, placeholder_delta, graph_period,
       encoding, decimator))
   end_stat_time = time.time()