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

Exact same behavior, but faster. I also think it's more readable.
diff --git a/system/libraries/Javascript.php b/system/libraries/Javascript.php
index 542a0ec..7f1d855 100644
--- a/system/libraries/Javascript.php
+++ b/system/libraries/Javascript.php
@@ -737,7 +737,7 @@
 	{
 		// JSON data can optionally be passed to this function
 		// either as a database result object or an array, or a user supplied array
-		if ( ! is_null($result))
+		if ($result !== NULL)
 		{
 			if (is_object($result))
 			{
@@ -823,7 +823,7 @@
 	 */
 	protected function _prep_args($result, $is_key = FALSE)
 	{
-		if (is_null($result))
+		if ($result === NULL)
 		{
 			return 'null';
 		}