Fix casting Encoding without style field

[juantascon]

fixes an issue found when parsing Encodings without the style field

example:

              "post": {
                "operationId": "TransferMtData",
                "requestBody": {
                  "content": {
                    "multipart/related": {
                      "encoding": {
                        "binaryMtData": {
                          "contentType": "application/vnd.3gpp.5gnas",
                          "headers": {
                            "Content-Id": {
                              "schema": {
                                "type": "string"
                              }
                            }

produces the following error:

** (FunctionClauseError) no function clause matching in String.to_atom/1    
    
    The following arguments were given to String.to_atom/1:
    
        # 1
        nil
    
    Attempted function clauses (showing 1 out of 1):
    
        def to_atom(+string+) when -is_binary(string)-
    
    (elixir 1.13.0-dev) lib/string.ex:2436: String.to_atom/1
    (open_api_spex 3.11.0) lib/open_api_spex/open_api/decode.ex:94: OpenApiSpex.OpenApi.Decode.update_map_if_key_present/3
    (open_api_spex 3.11.0) lib/open_api_spex/open_api/decode.ex:351: anonymous fn/1 in OpenApiSpex.OpenApi.Decode.to_struct/2
    (elixir 1.13.0-dev) lib/enum.ex:1595: anonymous fn/3 in Enum.map/2
    (stdlib 3.15.1) maps.erl:410: :maps.fold_1/3
    (elixir 1.13.0-dev) lib/enum.ex:2406: Enum.map/2
    (elixir 1.13.0-dev) lib/map.ex:219: Map.new/2
    (elixir 1.13.0-dev) lib/map.ex:813: Map.update!/3

[mbuhot]

👍 thanks @juantascon

[zoten]

Hi everyone!
Any chance for this to be merged? Are there any blockers?

Thanks!

[mbuhot]

Thanks for the reminder @zoten!