Fix implicit nullable parameters

src/Driver/ParallelFilesystemDriver.php

@@ -31,7 +31,7 @@ final class ParallelFilesystemDriver implements FilesystemDriver
     /**
      * @param int       $workerLimit Maximum number of workers to use from the pool for open files.
      */
-    public function __construct(WorkerPool $pool = null, int $workerLimit = self::DEFAULT_WORKER_LIMIT)
+    public function __construct(?WorkerPool $pool = null, int $workerLimit = self::DEFAULT_WORKER_LIMIT)
     {
         $this->pool = $pool ?? workerPool();
         $this->workerLimit = $workerLimit;

src/FileCache.php

@@ -119,7 +119,7 @@ public function get(string $key): ?string
         }
     }
 
-    public function set(string $key, string $value, int $ttl = null): void
+    public function set(string $key, string $value, ?int $ttl = null): void
     {
         if ($ttl < 0) {
             throw new \Error("Invalid cache TTL ({$ttl}); integer >= 0 or null required");

src/FilesystemException.php

@@ -4,7 +4,7 @@
 
 class FilesystemException extends \Exception
 {
-    public function __construct(string $message, \Throwable $previous = null)
+    public function __construct(string $message, ?\Throwable $previous = null)
     {
         parent::__construct($message, 0, $previous);
     }