You can use dynamic SQL

I mean first check the version

Then build your SQL statement using a string variable, for example nvarchar(max) 

Then execute it by sp_executeSQL

I think following script can work for this task

    -- check for sql server version
    declare @sql nvarchar(max)
    if (select cast(left(cast(serverproperty('productversion') as varchar), 4) as decimal(5, 3))) >= 10 
    
    set @sql = N'CREATE unique nonclustered index ix1_table ON [table] (column1, column2)
            WHERE column1 is not null and column2 is not null'
    
            ELSE
    
    set @sql = N'CREATE nonclustered index ix1_table ON [table] (column1, column2)'
    
    
    exec sp_executeSQL @sql