Improve E_WARNING compliance

While running CiviCRM, every single page load produces dozens of PHP warnings, mostly due to unassigned variables or invalid array indexes. This is particularly troublesome in Drupal, where the default error-handler logs each error to the database, thus turning every single page load a write-intensive operation.

The attached patch affects all templates within the civicrm/templates/CRM directory tree, replacing
<code>

{if $foo}

</code>, and its semantic equivalents, with
<code>

{if !empty($foo)}

</code>