Take input as list which can be null

Hi

I have a parameter of type list that can be null. Here is the code-

(a as list)=>
    let 
        b = if a is null then {""} else a
    in 
        b

And I am invoking the above function like this-

let
    Source = Query1(null)
in
    Source

The error I get if I run the above query-

Expression.Error: We cannot convert the value null to type List.
Details:
    Value=
    Type=[Type]

Can someone please tell me how I can pass null to a parameter of type list.

Thanks
Vivek