allow setting key to null

currently even though `$this->key` is nullable, the setter cannot set the key to `null` because of the argument type

Author: blueo

src/DataTransferObjects/CacheKeyDto.php

@@ -13,7 +13,7 @@ public function getKey(): ?string
         return $this->key;
     }
 
-    public function setKey(string $key): void
+    public function setKey(?string $key): void
     {
         $this->key = $key;
     }