How to force a method to fall with an exception?

Question

ASP.NET

There is a repository method that adds an object to the base (in my case the base is a list). However, if an empty object is passed to it, it must be thrown with an exception for further processing.

public bool Save(T entity)
        {
            entity = new T();

            bool SaveResult = false;

            if (entity != null)
            {
                AllItem.Add(entity);

                SaveResult = true;
            }
            return SaveResult;
        }

And here is the test code

[TestMethod]
        public void SaveNullObjectTest()
        {
            AccountModel ent = new AccountModel();

            ent = null;

            bool result = AccountBS.Save(ent);

            Assert.IsFalse(result);
        }

But for some reason the result is still true

Answer 1

entity = new T();
Doesn't it bother you?
Object instance created, variable will not be null

Answer 2

public void Save(T entity)
{
    if (entity == null)
        throw new ArgumentNullException(nameof(entity));

    AllItem.Add(entity);
}