Improve code, fix filesystem test failing after first run

src/Adapter/NullAdapter.php

@@ -20,7 +20,7 @@ final class NullAdapter implements CacheAdapterInterface
      * @param int|null $ttl
      * @return void
      */
-    public function set(string $key, $value, int $ttl = null)
+    public function set(string $key, $value, int $ttl = null): void
     {
     }
 
@@ -28,7 +28,7 @@ final class NullAdapter implements CacheAdapterInterface
      * @param string $key
      * @return void
      */
-    public function delete(string $key)
+    public function delete(string $key): void
     {
     }