How to make Fill Down conditions?

In the solution below, the table is grouped on System, initially with option all rows.

Next I created a function AddFirstValueAndFillDown to perform the required operations and adjusted the previously generated code so the function is used for each subtable. I also changed type table in Value.Type(Source) to keep the data types from the Source columns.

let
    AddFirstValueAndFillDown = (Table as table) as table =>
    let
        #"Added Index" = Table.AddIndexColumn(Table, "Index", 0, 1),
        #"Added Custom" = Table.AddColumn(#"Added Index", "Custom", each if [Index] = 0 and [State] = null then 1 - List.First(List.Select(Table[State],each _ <> null)) else [State], Int64.Type),
        #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"State", "Index"}),
        #"Renamed Columns" = Table.RenameColumns(#"Removed Columns",{{"Custom", "State"}}),
        #"Filled Down" = Table.FillDown(#"Renamed Columns",{"State"})
    in
        #"Filled Down",

    Source = Table1,
    #"Grouped Rows" = Table.Group(Source, {"System"}, {{"AllData", AddFirstValueAndFillDown, Value.Type(Source)}}),
    #"Expanded AllData" = Table.ExpandTableColumn(#"Grouped Rows", "AllData", {"Date", "State"}, {"Date", "State"})

in
    #"Expanded AllData"