Merge - How to treat null value as All

I would like to merge my Flags table with my Data table matching across a number of columns (example below shows 3 matching criteria (Category, Project and SubProject).  The only difference to a traditional merge is that if in the Flag table the field is blank/null then this should be treated as wildcard for all, or just ignored.

Flag Table

Data Table

Desired Output

Currently I try like this, but it does not return expected result, any guidance would be much appreciated.

let
    Source = Excel.CurrentWorkbook(){[Name="Data"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Category", type text}, {"Project", type text}, {"SubProject", Int64.Type}}),
    Buffer=Table.Buffer(Flags),
    #"Added Custom" = Table.AddColumn(#"Changed Type","Flag",(i)=>Table.SelectRows(Buffer, each 
    (i[Category] = [Category] or i[Category] = null) and (i[Project] = [Project] or i[Project] = null) and (i[SubProject] = [SubProject] or i[SubProject] = null)
      
) [Flag]),
    #"Expanded data" = Table.ExpandListColumn(#"Added Custom", "Flag")
in
    #"Expanded data"

I intend to apply this to tables of data with circa 100K rows, so if this is also not the most efficient approach I am open to completey different approaches.