[elixir] Typespec - allow null on optional struct-attributes

[mrmstn]

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• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

This PR allows null-values for struct-attributes which are not marked as required. This is very important since the compiler would mark this as a error since a null value in a struct would break the typespec contract.

With this small typespec fix, non-required and nullable attributes will be markes as possible nil values for the given model

[wing328]

@mrmstn thanks for the PR. Shall we also use the nullable mustache tag in the Elixir client? (we can definitely do it in another PR instead)

Ref:

• https://github.com/OpenAPITools/openapi-generator/pull/1059/files#diff-0909ca3c214075716264f5878b273ee6R46
• https://github.com/OpenAPITools/openapi-generator/pull/1073/files#diff-37c4b71d084f33d24c1b198a37922680R67

[mrmstn]

I think this would be a good idea - I kind of missed the existance of nullable all alonge 🙈

[mrmstn]

I also added the nullable type to the musache file.

Based on the following swagger 2.0 defintion

definitions:
  Entity:
    type: object
    properties:
      requiredDate:
        type: string
        format: date
        x-nullable: false
      notRequiredDate:
        type: string
        format: date
        x-nullable: false
      nullableRequiredDate:
        type: string
        format: date
        x-nullable: true
      nullableNotRequiredDate:
        type: string
        format: date
        x-nullable: true
    required:
      - requiredDate
      - nullableRequiredDate

this PR will result to the following results

defmodule SwaggerPetstore.Model.Entity do
  @moduledoc """
  
  """

  @derive [Poison.Encoder]
  defstruct [
    :"requiredDate",
    :"notRequiredDate",
    :"nullableRequiredDate",
    :"nullableNotRequiredDate"
  ]

  @type t :: %__MODULE__{
    :"requiredDate" => Date.t,
    :"notRequiredDate" => Date.t | nil,
    :"nullableRequiredDate" => Date.t | nil,
    :"nullableNotRequiredDate" => Date.t | nil
  }
end

As you can see, even if you didn't allow a non required attribute to be null, the typespec will mark the attribute as possible null. This is given to the fact that elixir will initiate all attributes in a struct as null and therefore a "optional" attribute in a struct can only be null and not "not present"

[wing328]

LGTM