Revert "Fixed return values types in session drivers"

This reverts commit 5fa4b7266d7c61dc2482e529bd6afb9a67edadb7.
diff --git a/system/libraries/Session/drivers/Session_files_driver.php b/system/libraries/Session/drivers/Session_files_driver.php
index 217d3f9..d3ef34a 100644
--- a/system/libraries/Session/drivers/Session_files_driver.php
+++ b/system/libraries/Session/drivers/Session_files_driver.php
@@ -156,13 +156,13 @@
 				if (($this->_file_handle = fopen($this->_file_path.$session_id, 'w+b')) === FALSE)
 				{
 					log_message('error', "Session: File '".$this->_file_path.$session_id."' doesn't exist and cannot be created.");
-					return '';
+					return FALSE;
 				}
 			}
 			elseif (($this->_file_handle = fopen($this->_file_path.$session_id, 'r+b')) === FALSE)
 			{
 				log_message('error', "Session: Unable to open file '".$this->_file_path.$session_id."'.");
-				return '';
+				return FALSE;
 			}
 
 			if (flock($this->_file_handle, LOCK_EX) === FALSE)
@@ -170,7 +170,7 @@
 				log_message('error', "Session: Unable to obtain lock for file '".$this->_file_path.$session_id."'.");
 				fclose($this->_file_handle);
 				$this->_file_handle = NULL;
-				return '';
+				return FALSE;
 			}
 
 			// Needed by write() to detect session_regenerate_id() calls
diff --git a/system/libraries/Session/drivers/Session_memcached_driver.php b/system/libraries/Session/drivers/Session_memcached_driver.php
index 80d8b3d..600b8ca 100644
--- a/system/libraries/Session/drivers/Session_memcached_driver.php
+++ b/system/libraries/Session/drivers/Session_memcached_driver.php
@@ -170,7 +170,7 @@
 			return $session_data;
 		}
 
-		return '';;
+		return FALSE;
 	}
 
 	// ------------------------------------------------------------------------
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;
 	}
 
 	// ------------------------------------------------------------------------