//Object itself.
 RuleFor(x => x).NotNull();

[Test]
public void RequestObjectIsNull_ExpectError()
{
    BusinessRequest request = null;
    var result = validator.Validate(request);

    Assert.IsFalse(result.IsValid);
}

public class MyValidator : AbstractValidator<MyObject>
    {
        public MyValidator()
        {
            RuleSet("MyRule", () =>
            {
                RuleFor(x=>x.MyProperty=="Something").WithMessage("failed");
            });
        }

        public override Task<ValidationResult> ValidateAsync(ValidationContext<MyObjec>> context, CancellationToken cancellation = default)
        {
            return context.InstanceToValidate == null ? Task.FromResult(new ValidationResult(new[] { new ValidationFailure("MyObject", "filed with null") })) : base.ValidateAsync(context, cancellation);
        }
    }