Manually implement PR #2033

Check for an empty encryption_key shouldn't use strict comparison.
diff --git a/system/libraries/Session/drivers/Session_cookie.php b/system/libraries/Session/drivers/Session_cookie.php
index 3c48482..6d926ae 100644
--- a/system/libraries/Session/drivers/Session_cookie.php
+++ b/system/libraries/Session/drivers/Session_cookie.php
@@ -219,7 +219,7 @@
 				: $this->CI->config->item($key);
 		}
 
-		if ($this->encryption_key === '')
+		if (empty($this->encryption_key))
 		{
 			show_error('In order to use the Cookie Session driver you are required to set an encryption key in your config file.');
 		}