fix(http2): set default initial window size to 65535 if not specified

Author: KoenRijpstra

packages/grpc-js/src/transport.ts

@@ -719,7 +719,7 @@ export class Http2SubchannelConnector implements SubchannelConnector {
         settings: {
           initialWindowSize:
             options['grpc-node.flow_control_window'] ??
-            http2.getDefaultSettings().initialWindowSize,
+            http2.getDefaultSettings().initialWindowSize ?? 65535,
         }
       });