Fix Null memcache fallback to match interface

lib/private/memcache/null.php

@@ -22,7 +22,7 @@
 
 namespace OC\Memcache;
 
-class Null extends Cache {
+class Null extends Cache implements \OCP\IMemcache {
 	public function get($key) {
 		return null;
 	}
@@ -39,11 +39,15 @@ class Null extends Cache {
 		return true;
 	}
 
-	public function inc($key) {
+	public function add($key, $value, $ttl = 0) {
 		return true;
 	}
 
-	public function dec($key) {
+	public function inc($key, $step = 1) {
+		return true;
+	}
+
+	public function dec($key, $step = 1) {
 		return true;
 	}