fix: explicit nullable type (#41)

src/Builder.php

@@ -31,7 +31,7 @@ class Builder extends BaseBuilder
      */
     protected $connection = Connection::DEFAULT_NAME;
 
-    public function __construct(Client $client = null)
+    public function __construct(?Client $client = null)
     {
         $this->grammar = new Grammar();
         $this->client = $client ?? $this->getThisClient();