What is NoOpMediator in Clean Architecture

Question

I am playing around with the Clean Architecture by Ardalis.

Can someone please explain what this NoOpMediator role in Unit testing?

using System.Runtime.CompilerServices;
using MediatR;

namespace Clean.Architecture.UnitTests;

public class NoOpMediator : IMediator
{
  public Task Publish(object notification, CancellationToken cancellationToken = default)
  {
    return Task.CompletedTask;
  }

  ...
  ...
}

It does not seem to do anything. For four of its methods it returns Task.CompletedTask. And for the rest of the two it returns Task.FromResult<object?>(default)

Further more in the BaseEfRepoTestFixture class here, there is no configuration of Mock object. Bit misty.

Also noted that this is configured in CustomWebApplicationFactory way down here.

services.AddScoped<IMediator, NoOpMediator>();

Even if I comment out that line, the tests still pass.

There should be an example test to show how this works.