config_item() to return NULL instead of FALSE for non-existing items

Close #3001
Close #3232
Related: #3244
diff --git a/system/libraries/Encrypt.php b/system/libraries/Encrypt.php
index 995bf0b..1af42ed 100644
--- a/system/libraries/Encrypt.php
+++ b/system/libraries/Encrypt.php
@@ -111,7 +111,7 @@
 
 			$key = config_item('encryption_key');
 
-			if ($key === FALSE)
+			if ( ! strlen($key))
 			{
 				show_error('In order to use the encryption class requires that you set an encryption key in your config file.');
 			}