Don't display disk usage if it hasn't been calculated yet

Signed-off-by: Richard Mansfield <richardm@catalyst.net.nz>

htdocs/theme/raw/templates/admin/stats.tpl

@@ -23,7 +23,9 @@ addLoadEvent(function () {literal}{{/literal}
     <p>&nbsp;{$sitedata.viewsperuser}</p>
     {/if}
     <p><strong>{str tag=databasesize section=admin}:</strong> {$sitedata.dbsize|display_size}</p>
+    {if $sitedata.diskusage}
     <p><strong>{str tag=diskusage section=admin}:</strong> {$sitedata.diskusage|display_size}</p>
+    {/if}
     <p><strong>{str tag=maharaversion section=admin}:</strong> {$sitedata.release}</p>
     <p><strong>{str tag=Cron section=admin}:</strong> {if $sitedata.cronrunning}{str tag=runningnormally section=admin}{else}{str tag=cronnotrunning section=admin}{/if}</p>
   </div>