Remove hyphens when 'tel' extra var is empty

cf. https://xetown.com/topics/1356877

classes/extravar/Extravar.class.php

@@ -322,7 +322,7 @@ class ExtraItem
 				return ($value) ? sprintf('<a href="mailto:%s">%s</a>', $value, $value) : "";
 
 			case 'tel' :
-				return sprintf('%s-%s-%s', $value[0], $value[1], $value[2]);
+				return $value ? implode('-', $value) : '';
 				
 			case 'textarea' :
 				return nl2br($value);