Power Query Add rows of zero amount

Here is a sample code that uses similar logic to your example.

let
    Source = 
    Table.FromRows(
        Json.Document(
            Binary.Decompress(
                Binary.FromText("i45WCkgsKlFwVNJRMtE31DcyMDIDMoFErA5UygnIN0ZImZsrxcYCAA==", BinaryEncoding.Base64), 
                Compression.Deflate
            )
        ), 
        let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Part = _t, Date = _t, Qty = _t]
    ),
    #"Changed Type" = 
    Table.TransformColumnTypes(
        Source,
        {
            {"Part", type text}, {"Date", type date}, {"Qty", Int64.Type}
        }
    ),
    generate_records = 
    Table.AddColumn(
        #"Changed Type", 
        "next12Months", 
        each 
        List.Transform(
            {0..12}, 
            (r)=> [
                Part=[Part], 
                Date=Date.AddMonths([Date], r), 
                Qty=if r = 0 then [Qty] else 0
            ]
        ), 
        type list
    ),
    remove_columns = 
    Table.RemoveColumns(
        generate_records,
        {"Part", "Date", "Qty"}
    ),
    expand_lists = 
    Table.ExpandListColumn(
        remove_columns, 
        "next12Months"
    ),
    expand_records = 
    Table.ExpandRecordColumn(
        expand_lists, 
        "next12Months", 
        {"Part", "Date", "Qty"}, 
        {"Part", "Date", "Qty"}
    ),
    set_types = 
    Table.TransformColumnTypes(
        expand_records,
        {
            {"Part", type text}, {"Date", type date}, {"Qty", Int64.Type}
        }
    )
in
    set_types

Here I start with example table...

and end with...