do not load empty custom_client_config

src/flutter_ffi.rs

@@ -2124,8 +2124,10 @@ pub mod server_side {
             *config::APP_DIR.write().unwrap() = app_dir.into();
         }
         if let Ok(custom_client_config) = env.get_string(&custom_client_config) {
+            if !custom_client_config.is_empty() {
                 crate::read_custom_client(custom_client_config);
             }
+        }
         std::thread::spawn(move || start_server(true));
     }