Fix unless code comment (#53529)

The existing code comment was copied from `when`, and therefore mistakenly explains the callback is called when the value is truthy.

Author: fritz-c

src/Illuminate/Collections/Enumerable.php

@@ -334,7 +334,7 @@ public function whenEmpty(callable $callback, ?callable $default = null);
     public function whenNotEmpty(callable $callback, ?callable $default = null);
 
     /**
-     * Apply the callback if the given "value" is (or resolves to) truthy.
+     * Apply the callback if the given "value" is (or resolves to) falsy.
      *
      * @template TUnlessReturnType
      *