Display blank instead of 'user0' in Assigned to field

When a user lacking the 'update_bug_assign_threshold' privilege (e.g.
updater) tries to edit an issue that has no handler, the 'Assigned to'
field shows 'user0' instead of a blank field as expected.

Fixes #12480

bug_update_advanced_page.php

@@ -139,7 +139,11 @@
 $tpl_additional_information_textarea = $tpl_show_additional_information ? string_textarea( $tpl_bug->additional_information ) : '';
 $tpl_show_steps_to_reproduce = in_array( 'steps_to_reproduce', $t_fields );
 $tpl_steps_to_reproduce_textarea = $tpl_show_steps_to_reproduce ? string_textarea( $tpl_bug->steps_to_reproduce ) : '';
-$tpl_handler_name = string_display_line( user_get_name( $tpl_bug->handler_id ) );
+if( NO_USER == $tpl_bug->handler_id ) {
+	$tpl_handler_name =  '';
+} else {
+	$tpl_handler_name = string_display_line( user_get_name( $tpl_bug->handler_id ) );
+}
 
 $tpl_can_change_view_state = $tpl_show_view_state && access_has_project_level( config_get( 'change_view_status_threshold' ) );