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

Exact same behavior, but faster. I also think it's more readable.
diff --git a/system/database/DB_result.php b/system/database/DB_result.php
index dfd8081..a044fd5 100644
--- a/system/database/DB_result.php
+++ b/system/database/DB_result.php
@@ -354,7 +354,7 @@
 			return;
 		}
 
-		if ($key !== '' && ! is_null($value))
+		if ($key !== '' && $value !== NULL)
 		{
 			$this->row_data[$key] = $value;
 		}