Use String default ctor instead of empty string

src/buffer.hh

@@ -195,7 +195,7 @@ public:
     ValueMap& values() const { return m_values; }
 
     void run_hook_in_own_context(StringView hook_name, StringView param,
-                                 String client_name = "");
+                                 String client_name = {});
 
     void reload(StringView data, timespec fs_timestamp = InvalidTime);