Main Joomla template should omit the pageTitle element if it is not defined for that page

In joomla.tpl - we include an <h1> tag for $pageTitle. However, for the Event Info pages (civicrm/event/info) - we don't set a value for pageTitle. The reason for this exception is that the Event Title is rendered within the "vevent" block to comply with that micro-format. This doesn't cause a problem in Drupal because the drupal theme's omit the element when the title isn't defined.

We should update common/Joomla.tpl to make this element conditional - and thereby match the behavior in Drupal pages.