Compare Null To Null

I want to apply something like below in my where clause:

select users.first_name

from users
where users.somecolumn = isnull(@somecolumnvalue, users.somecolumn)


The idea is to get all the values from users table if the value contained in @somecolumnvalue is null.
Now the issue is, users.somecolumn is nullable and has many rows that have null values. This query returns only those rows that have values populated in users.somecolumn.
How can the desired result be achieved?