SQL SSRS and Default Values Stored Procedure
You could handle nulls inside the stored procedure with an isnull() like this:

create procedure rpt.FinanceReport
    @AccountYearParam int 
as

set @AccountYearParam = isnull(@AccountYearParam, 2018)
--
select * from FinanceTable where AccountYear = @AccountYearParam


and if you want to get really fancy, you can use this (it will return the current year):

create procedure rpt.FinanceReport
    @AccountYearParam int 
as

set @AccountYearParam = isnull(@AccountYearParam, datepart(year, getdate()))
--
select * from FinanceTable where AccountYear = @AccountYearParam  


You can also handle it in the where clause, although this method might be considerably slower:

create procedure rpt.FinanceReport
    @AccountYearParam int 
as
--
select * from FinanceTable where AccountYear = isnull(@AccountYearParam, datepart(year, getdate()))