如何设置值是函数是空的？

In my functions.php for Wordpress I've created a Placeholder to use with Events Manager Pro. (Basically it gets the ticket prices for events on my blogs.)

It works, but I need an addition to is and I cannot seem to get it to work.

I need a condition added:

If #_PRICE (this function) = empty (null) then set the value to '0.00'

The current code is:

<?php
function em_event_output_placeholder_tax($replace, $EM_Event, $result){
switch( $result ){
    case '#_PRICE':
        $replace = ''; 
        foreach( $EM_Event->get_tickets()->tickets as $EM_Ticket ){
        $replace .= number_format($EM_Ticket->get_price(), 2, '.', '');
     }
        break;
}
return $replace;
}
add_filter('em_event_output_placeholder','em_event_output_placeholder_tax',1,3);
?>

Please help...