Why is “null” present in C# and java?

static string AcceptNotNullObject(object! s)
{
    return s.ToString();
}

MyResource resource;
try
{
  resource = new MyResource();
  //
  // Do some work
  //
}
finally
{
  if (resource != null)
    resource.Close();
}

let counter = ref 0;;
let next_counter_value () = (counter := !counter + 1; !counter);;

type distributed_computation_result = NotYetAvailable | Result of float;;
let print_result r = match r with
    | Result(f) -> Printf.printf "result is %fn" f
    | NotYetAvailable -> Printf.printf "result not yet availablen";;

class A {
  B fieldb;
}

class B {
  A fielda;
}

A a = new A() // a.fieldb is null
B b = new B() { fielda = a } // b.fielda isnt
a.fieldb = b // now it isnt null anymore