Replace is_null() with === / !== NULL

Exact same behavior, but faster. I also think it's more readable.
diff --git a/system/libraries/Xmlrpc.php b/system/libraries/Xmlrpc.php
index c3c7690..9e60791 100644
--- a/system/libraries/Xmlrpc.php
+++ b/system/libraries/Xmlrpc.php
@@ -368,7 +368,7 @@
 	 */
 	public function timeout($seconds = 5)
 	{
-		if ( ! is_null($this->client) && is_int($seconds))
+		if ($this->client !== NULL && is_int($seconds))
 		{
 			$this->client->timeout = $seconds;
 		}