[Kotlin] fix #20231, OkHttp client can handle a field with a list of files

[jorgeferdz]

This PR is basically the same one as created a few days ago (#20232) but it removes the following line inside one of the new addPartToMultiPart methods:

if (obj == null) return

This way, the behavior when adding null values is still the same.

issue: Kotlin OkHttp client does not process correctly when a field in the endpoint is defined as an array of lists like this:

...
requestBody:
  content:
    multipart/form-data:
      schema:
        type: object
        properties:
          attachments:
            type: array
            description: attached files
            items:
              type: string
              format: binary
...

If the autogenerated client is used, the server will receive always 1 item in the list but it is not a file or anything and it is trying to serialize like a normal object.

I noticed this issue while developing my own project, so I modified this code (by looking at the Java counterpart that can be seen here) and tested it by building the project in my local machine and adding the plugin to my project manually.

I tried to add some testing (I added the faulty specification) but I was not really sure how to do it. Also by reading the documentation, it didn't find it very clearly what I should do. Any guidance regarding how to add test for that would be very much appreciated.

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@wing328 I created the tests and I will add them in a separate PR so it doesn't becomes too big.