Merge - How to treat null value as All

After some experimenting seems I was pretty close, seems I had my logic wrong way round, not sure I unerstand it yet, but this is returning expected results

let
    Source = Excel.CurrentWorkbook(){[Name="Data"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Category", type text}, {"Project", type text}, {"SubProject", type text}}),
    Buffer=Table.Buffer(Flags),
    #"Added Custom" = Table.AddColumn(#"Changed Type","Flag",(i)=>Table.SelectRows(Buffer, each

    ([Category] = i[Category] or [Category] = null) and ([Project] = i[Project] or [Project] = null) and ([SubProject] = i[SubProject] or [SubProject] = null)
    
    ) [Flag]),
    #"Expanded data" = Table.ExpandListColumn(#"Added Custom", "Flag")
in
    #"Expanded data"