Use this instead:
select * from test where iif(empty(x),.t.,f1$x)
select * from test where iif(x='none',.t.,f1$x)

>Is this normal VFP (7 or 8) Select-SQL behavior?
>

>create cursor test (f1 i)
>for i = 1 to 10
>	insert into test values (i)
>endfor
>x='1,2,3,4,5'
>*!*	x=''  && uncomment to cause an error
>select * from test where iif(empty(x),.t.,f1 in (&x))
>*!*	x='none'  && uncomment to cause an error
>select * from test where iif(x='none',.t.,f1 in (&x))
>