Remove null from streaming finishReason

[guersam]

Currently, initial response chunks from chat completion emits null at choices[*].finishReason:

ChatChunkResponse(chatcmpl-8OGfnT5mmJwiTTmICOgi26fOxrJnf,List(Choices(Delta(Some(Assistant),Some(),None,None),null,0)),1700794655,gpt-4-1106-preview,chat.completion.chunk,Some(fp_a24b4d720c))
<FAIL>: Die(java.lang.NullPointerException: Cannot invoke "String.length()" because the return value of "sttp.openai.requests.completions.chat.ChatChunkRequestResponseData$Choices.finishReason()" is null,Stack trace for thread "zio-fiber-":
)

This PR makes it optional.

[guersam]

Just noticed that FunctionTollCall.id should be optional as well in streaming mode.