Decoder.elm

optional "note" string            ""
optional "note" (map Just string) Nothing 
optional "note" (map (\x -> if x == "" then Nothing else Just x) string) Nothing

Encoder.elm

import Json.Encode as Json

type alias NotePayload = { notes : Maybe String }

-- { "notes": "abc" } 
encodeNotePayload : NotePayload -> Value
encodeNotePayload payload =
  Json.object [ ("notes", encodeMaybeString payload.notes ) ]
  
encodeMaybeString : Maybe String -> Value
encodeMaybeString maybeString =
  case maybeString of
    Nothing -> Json.null
    Just str -> Json.string str