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

Exact same behavior, but faster. I also think it's more readable.
diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index c405eb6..bbd0b52 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -511,7 +511,7 @@
 	{
 		foreach ($this->_field_data as $field => $row)
 		{
-			if ( ! is_null($row['postdata']))
+			if ($row['postdata'] !== NULL)
 			{
 				if ($row['is_array'] === FALSE)
 				{
@@ -583,7 +583,7 @@
 
 		// If the field is blank, but NOT required, no further tests are necessary
 		$callback = FALSE;
-		if ( ! in_array('required', $rules) && is_null($postdata))
+		if ( ! in_array('required', $rules) && $postdata === NULL)
 		{
 			// Before we bail out, does the rule contain a callback?
 			if (preg_match('/(callback_\w+(\[.*?\])?)/', implode(' ', $rules), $match))
@@ -598,7 +598,7 @@
 		}
 
 		// Isset Test. Typically this rule will only apply to checkboxes.
-		if (is_null($postdata) && $callback === FALSE)
+		if ($postdata === NULL && $callback === FALSE)
 		{
 			if (in_array('isset', $rules, TRUE) OR in_array('required', $rules))
 			{