[ticket/13782] Rename null log to dummy for PHP7 compatibility

PHPBB3-13782

phpBB/phpbb/log/dummy.php

@@ -14,9 +14,9 @@
 namespace phpbb\log;
 
 /**
-* Null logger
+* Dummy logger
 */
-class null implements log_interface
+class dummy implements log_interface
 {
 	/**
 	* {@inheritdoc}