fix(http2): handle default initial window size correctly

Author: KoenRijpstra

packages/grpc-js/src/transport.ts

@@ -724,7 +724,7 @@ export class Http2SubchannelConnector implements SubchannelConnector {
       });
 
       // Send WINDOW_UPDATE now to avoid 65 KB start-window stall.
-      const defaultWin = http2.getDefaultSettings().initialWindowSize; // 65 535 B
+      const defaultWin = http2.getDefaultSettings().initialWindowSize ?? 65535; // 65 535 B
       const connWin = options[
         'grpc-node.connection_flow_control_window'
       ] as number | undefined;