Isnull() syntax is built in for this kind of thing.
declare @Int int = null;
declare @Values table ( id int, def varchar(8) )
insert into @Values values (8, 'I am 8');
-- fails
select *
from @Values
where id = @Int
-- works fine
select *
from @Values
where id = isnull(@Int, 8);
For your example keep in mind you can change scope to be yet another where predicate off of a different variable for complex boolean logic. Only caveat is you need to cast it differently if you need to examine for a different data type. So if I add another row but wish to specify int of 8 AND also the reference of text similar to 'repeat' I can do that with a reference again back to the 'isnull' of the first variable yet return an entirely different result data type for a different reference to a different field.
declare @Int int = null;
declare @Values table ( id int, def varchar(16) )
insert into @Values values (8, 'I am 8'), (8, 'I am 8 repeat');
select *
from @Values
where id = isnull(@Int, 8)
and def like isnull(cast(@Int as varchar), '%repeat%')