fix: allow empty content for Response

[MichalDolata]

#451 introduced a change that doesn't meet OpenAPI specification. https://spec.openapis.org/oas/v3.1.0#fixed-fields-14 - content can be empty.

The mentioned change caused errors in following example

  operation(:operation,
    summary: "summary",
    description: "description",
    request_body: {nil, "application/json", SchemaRequest},
    responses: %{
      201 => {"Operation succeed", nil, nil}
    }
  )

** (RuntimeError) Expected string or map for request_body: nil
    (open_api_spex 3.11.0) lib/open_api_spex/operation.ex:320: OpenApiSpex.Operation.build_content_map/2
    (open_api_spex 3.11.0) lib/open_api_spex/operation.ex:161: OpenApiSpex.Operation.response/4
    (open_api_spex 3.11.0) lib/open_api_spex/operation_builder.ex:82: anonymous fn/1 in OpenApiSpex.OperationBuilder.build_responses/1
    (elixir 1.13.1) lib/enum.ex:1597: anonymous fn/3 in Enum.map/2
    (stdlib 3.17) maps.erl:410: :maps.fold_1/3
    (elixir 1.13.1) lib/enum.ex:2408: Enum.map/2
    (elixir 1.13.1) lib/map.ex:219: Map.new/2
    (open_api_spex 3.11.0) lib/open_api_spex/controller_specs.ex:393: OpenApiSpex.ControllerSpecs.operation_spec/3
    ...: (module)

This PR allows back to define empty response content

[moxley]

Nice catch @MichalDolata! Looks good. Thank you!!!