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

Exact same behavior, but faster. I also think it's more readable.
diff --git a/system/libraries/Table.php b/system/libraries/Table.php
index 8656990..b77fcf1 100644
--- a/system/libraries/Table.php
+++ b/system/libraries/Table.php
@@ -291,7 +291,7 @@
 	{
 		// The table data can optionally be passed to this function
 		// either as a database result object or an array
-		if ( ! is_null($table_data))
+		if ($table_data !== NULL)
 		{
 			if (is_object($table_data))
 			{