php: 5.6 Support

This addresses an issue with v1.14 where `??` (null coalescing operator) in
class Export throws a fatal error when using PHP 5.6. This is an issue
especially for people who cannot upgrade PHP but want to take advantage of
the latest osTicket codebase/features. This updates the null coalescing
operator to an isset ternary operator to keep backwards compatibility with
PHP 5.6.

include/class.export.php

@@ -928,8 +928,8 @@ function addTask($task, $zip, $prefix, $notes=true, $psize=null) {
     function download($options = array()) {
         global $thisstaff;
 
-        $notes = @$options['notes'] ?? false;
-        $tasks = @$options['tasks'] ?? false;
+        $notes = isset(@$options['notes']) ? @$options['notes'] : false;
+        $tasks = isset(@$options['tasks']) ? @$options['tasks'] : false;
 
         // TODO: Use a streaming ZIP library
         $zipfile = tempnam(sys_get_temp_dir(), 'zip');