Revert "Fixed return values types in session drivers"

This reverts commit 5fa4b7266d7c61dc2482e529bd6afb9a67edadb7.
diff --git a/system/libraries/Session/drivers/Session_redis_driver.php b/system/libraries/Session/drivers/Session_redis_driver.php
index af427d2..c3c75b3 100644
--- a/system/libraries/Session/drivers/Session_redis_driver.php
+++ b/system/libraries/Session/drivers/Session_redis_driver.php
@@ -171,7 +171,7 @@
 			return $session_data;
 		}
 
-		return '';
+		return FALSE;
 	}
 
 	// ------------------------------------------------------------------------