How to make Entity Framework only filter data when query fields are not null?

I have an mvc .net core application where the user is displayed some data, and can filter that data based on some input that he/her gives.
If no filters/constraints are given, then the while set of entities should jsut be returned.

I found this example, and found the second answer to be quite neat, with regards to what I want to do.

So I added this thing at the bottom of my controller:

public static class QueryableEx
{
    public static IQueryable<T> Where<T>(
        this IQueryable<T> @this,
        bool condition,
        Expression<Func<T, bool>> @where)
    {
        return condition ? @this.Where(@where) : @this;
    }
}

And then made this controller action which filters by one of three possible inputs:

[HttpPost]
public IActionResult query(QueryModel query)
{
    List<CustomerModel> customers = new List<CustomerModel>();

    var db = new DemoContext();
    customers = db.Customers 
        .Where(query.Name != null, x => x.Name == query.Name)
        .Where(query.Surname != null, x => x.Surname == query.Surname)
        .Where(query.Age != null, x => x.Age == query.Age)
            .ToList(); 
    
    
    return View("Index", customers);
}

This works like a charm, If I input a certain name, then I only get the results with that name and vice versa.

There is an issue though. If all of the input fields are null, then everything is filtered out. I want the opposite to happen, if no filters have been entered, then just return everything.

How do I do this? Making sure that no filtering happens if all the input fields are empty?

EDIT

By request, I here is the model I use for queries

public class QueryModel
{
    public string Name {get;set; }
    
    public string Surname { get; set; }
    
    public uint Age { get; set; }
    
    
}

And here is the customer one:

public class CustomerModel
{
        public int Id{get;set; }
        [Required]
        public string Name {get;set; }
        [Required]
        public string Surname { get; set; }
        [Required]
        [Range(18,110)]
        public uint Age { get; set; }
        [Required]
        public virtual AdressModel Adress { get; set; }
        [Required]
        public  DateTime Created { get; set; }
        [Required]
        public virtual List<PurchaseModel> purchases { get; set; }
}

>Solution :

Your model parameters are not nullable, so I suspect that you’ll end up looking for customers with Age equal Zero, hence no results.

Try:

customers = db.Customers 
    .Where(query.Name != default, x => x.Name == query.Name)
    .Where(query.Surname != default, x => x.Surname == query.Surname)
    .Where(query.Age != default, x => x.Age == query.Age)
        .ToList(); 

Change ‘null’ to ‘default’ in each case.