skip query step if condition is met

Thanks for the help. I found the solution I was looking for. The script should look like the following:

let

    Source =

        if List.Single(DateList) = null then

            Excel.Workbook(File.Contents(""C:\Mydocuments\Test.csv"), null, true)

        else

            null, // Preventing further steps if Source is null

    Output =

        if Source = null then

            #table({}, {}) // Returns an empty table

        else

            let

#"Promoted Headers" = Table.PromoteHeaders(Source, [PromoteAllScalars=true]),
#"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"Date", Int64.Type}, {"Code", Int64.Type}, {"Value", Int64.Type}})

           in

                #"Changed Type"in

    Output