Optional value where value is an enum generates warning that breaks compilation in case of --warnings-as-errors

[fusillicode]

The following schema

   {
      "name": "foo",
      "type": [
        "null",
        {
          "type": "record",
          "name": "Foo",
          "fields": [
            {
              "name": "value",
              "type": {
                "type": "enum",
                "name": "FooValue",
                "symbols": [
                  "Bar",
                  "Unknown"
                ]
              },
              "default": "Unknown"
            }
          ]
        }
      ],
      "default": "null"
    }

generates the following warning

Compiling 207 files (.ex)
    warning: this clause cannot match because a previous clause at line 29 always matches
    │
 35 │   def from_avro_map(%{} = invalid) do
    │   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/generated/foo.ex:35

The warning raises from the following generated code:

 @impl true
  def from_avro_map(%{} = avro_map) do
    value = avro_map["value"] || "Unknown"
    {:ok, %__MODULE__{value: decode_value(value)}}
  end

  @required_keys MapSet.new([])
  def from_avro_map(%{} = invalid) do
    actual = Map.keys(invalid) |> MapSet.new()
    missing = MapSet.difference(@required_keys, actual) |> Enum.join(", ")
    {:error, "Missing keys: " <> missing}
  end

  def from_avro_map(_) do
    {:error, "Expected a map."}
  end

The second definition of from_avro_map should be probably omitted in this scenario.