Address code standards issues

src/Model/Storage/IStorage.php

@@ -19,12 +19,12 @@ interface IStorage
 	public static function get($ref);
 
 	/**
-	 * @brief Put data in backend as $ref. If $ref is null a new reference is created.
+	 * @brief Put data in backend as $ref. If $ref is not defiend a new reference is created.
 	 * @param string  $data  Data to save
 	 * @param string  $ref   Data referece. Optional.
 	 * @return string Saved data referece
 	 */
-	public static function put($data, $ref = null);
+	public static function put($data, $ref = "");
 
 	/**
 	 * @brief Remove data from backend
@@ -32,4 +32,4 @@ interface IStorage
 	 * @return boolean  True on success
 	 */
 	public static function delete($ref);
-}
+}