[Elixir] Fixes issue with maps/dictionary not present in payload

Created by: mrmstn

Currently, there is an issue with the deserializations of maps, which are null.

As an example, the given payload is the following

{
  "ID": "2b892741-1853-3ddd-382f-fc12e93b5174",
  "CreateTime": 1666707394061348343,
  "ModifyTime": 1666707394064897549
}

the schema definitions looks something like this

components:
  schemas:
    Evaluation:
      properties:
        CreateTime:
          format: int64
          type: integer
        FailedTGAllocs:
          additionalProperties:
            $ref: '#/components/schemas/AllocationMetric'
          type: object
        ID:
          type: string
        ModifyTime:
          format: int64
          type: integer
      type: object
    AllocationMetric:
      properties:
        AllocationTime:
          format: int64
          type: integer
      type: object

As you can see, FailedTGAllocs is a simple dictionary with the value type of AllocationMetric. The generator will produce something like this (which is correct):

defimpl Poison.Decoder, for: Example.Model.Evaluation do
  import Example.Deserializer

  def decode(value, options) do
    value
    |> deserialize(:FailedTGAllocs, :map, Example.Model.AllocationMetric, options)
  end
end

The main issue is now, since payload doesn't have the field FailedTGAllocs, the Map.new(...) call will receive nil as the first parameter, which results in a pattern match error. To prevent this, I've added the maybe_transform_map function, which simply returns nil for nil, and the decoded/transformed model for everything else.

Since I'm the only one in the technical committee, @wing328, could you take a look?

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