Uni lacks replacing with void

[jponge]

When writing code such as:

return vertx.createHttpServer()
      .requestHandler(router::handle)
      .listen(8080)
      .onItem().invoke(() -> logger.info("HTTP server listening on port 8080"))
      .replaceWith(Uni.createFrom().voidItem());

where the return value is expected to be a Uni<Void>, then we cannot simplify with replaceWithNull since it keeps the original parametric type, yet with a null value.

We should add replaceWithVoid, as in:

return vertx.createHttpServer()
      .requestHandler(router::handle)
      .listen(8080)
      .onItem().invoke(() -> logger.info("HTTP server listening on port 8080"))
      .replaceWithVoid();