#nullable enable

​

using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var b = new List<List<string>?>() {

            new (){ "Mr Siang Lee Khaw", null },

            null,

            null,

            null,

            new (){ "Dis Mount", null, null },

            null,

            new (){ "02100", "Munith" },

            null,

            new (){ "Germany" }

        };

        Console.WriteLine(string.Join(", ", 

          b?

          .Where(x => x != null)

          .Select(a => string.Join(" ", a.Where(i => !string.IsNullOrWhiteSpace(i)))))); // 1, 23 <-- NOT OK

    }

}