Adjusted return type for CI_DB_utility::backup to suggestions

Author: ostrolucky

system/database/DB_utility.php

@@ -417,7 +417,7 @@ public function backup($params = array())
 			return gzencode($this->_backup($prefs));
 		}
 
-		return '';
+		return;
 	}
 
 }

user_guide_src/source/database/utilities.rst

@@ -252,7 +252,7 @@ Class Reference
 	.. php:method:: backup([$params = array()])
 
 		:param	array	$params: An associative array of options
-		:returns:	string
+		:returns:	raw/(g)zipped SQL query string
 		:rtype:	string
 
 		Perform a database backup, per user preferences.