Fix count being called on null

PHP 7.2 requires arguments to count being an array or an instance of Countable.

This property when not initialized, is neither of these, it's null.

src/Handler/CurlFactory.php

@@ -16,7 +16,7 @@ use Psr\Http\Message\RequestInterface;
 class CurlFactory implements CurlFactoryInterface
 {
     /** @var array */
-    private $handles;
+    private $handles = [];
 
     /** @var int Total number of idle handles to keep in cache */
     private $maxHandles;