Union : Based on condition

A simple join is enough,

let
    Client = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcszPS1WK1YlWcoIxnGEMFxjDFcZwAzNiAQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Client List" = _t]),
    Monthly = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bc+7CsAgDIXhd8ksaGKjc29PIY5d2/ffWoQaOLoFPsLPKYXW577IEfvAXoLE79ZA1RXaRonaZB9FlibHRKSJdeQXZuiYCHRMFDomGTqx71HomCTodEkZOvhzzjv1BQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Client List" = _t, Month = _t, Count = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Monthly,{{"Month", type date}, {"Count", Int64.Type}}),
    #"LeftOuter Join" = Table.NestedJoin(Client, {"Client List"}, #"Changed Type", {"Client List"}, "joined", JoinKind.LeftOuter),
    #"Expanded joined" = Table.ExpandTableColumn(#"LeftOuter Join", "joined", {"Month", "Count"}),
    #"Changed Month" = let dt=List.Max(#"Changed Type"[Month]) in Table.ReplaceValue(#"Expanded joined", null, dt, Replacer.ReplaceValue, {"Month"}),
    #"Changed Count" = Table.ReplaceValue(#"Changed Month", null, 0, Replacer.ReplaceValue, {"Count"})
in
    #"Changed Count"