Why doesn't SQL Server use this filtered index for NULL values?

I have a USERS table with a clustered primary key on UserId and a filtered index which contains rows with NULL middle names:

 create table dbo.users
 (
     UserId int identity 
       constraint pk_users primary key,
     FirstName varchar(100),
     MiddleName varchar(100),
     LastName varchar(100),
 );
 
 create unique nonclustered index ix_users_without_middlenames 
     on dbo.users(userid, LastName)
     where MiddleName IS NULL;
 
 insert dbo.users(FirstName, MiddleName, LastName)
 values ('A', 'A', 'A'),
        ('B', 'B', 'B'),
        ('C', null, 'C'),
        ('D', null, 'D');

Why doesn't SQL Server use the filtered index for this query:

 select userid from dbo.users where MiddleName IS NULL;

It shows a TRIVIAL optimization level, but even this query:

 select userid 
 from dbo.users WITH (INDEX (ix_users_without_middlenames))
 where MiddleName IS NULL;

uses a bookmark lookup to test for null.