[10.x] Turn Enumerable unless() $callback parameter optional (#51701)

* turn Enumerable unless()  parameter optional

* nullable parameters declaration

Author: faissaloux

src/Illuminate/Collections/Enumerable.php

@@ -338,11 +338,11 @@ public function whenNotEmpty(callable $callback, callable $default = null);
      * @template TUnlessReturnType
      *
      * @param  bool  $value
-     * @param  (callable($this): TUnlessReturnType)  $callback
+     * @param  (callable($this): TUnlessReturnType)|null  $callback
      * @param  (callable($this): TUnlessReturnType)|null  $default
      * @return $this|TUnlessReturnType
      */
-    public function unless($value, callable $callback, callable $default = null);
+    public function unless($value, ?callable $callback = null, ?callable $default = null);
 
     /**
      * Apply the callback unless the collection is empty.