How do I check if a field is empty?
You get that error because $node is not one of the variables passed to views-view-fields.tpl.php, which receives:


$view: The view in use.
$fields: an array of $field objects.
$row: The raw result object from the query, with all data it fetched.


Basing on the description, $row could be used to retried the node object. I cannot tell you the content of that variable, but using print_r() from your template file can help to understand its content.