[Bug]: Parallel function calling does not work (completions API, not assistants)

What happened?

It all starts with the fact that rivet cannot handle the assistants message in case of parallel function calling = it contains no content and multiple tool calls:

{
  "role": "assistant",
  "content": null,
  "tool_calls": [
    {
      "id": "call_BC3h2tFBsCGp7HuVQTY05EHG",
      "type": "function",
      "function": {
        "name": "test_a",
        "arguments": "{}"
      }
    },
    {
      "id": "call_1Y9o2pEdCgx0dYcw2R2hZ4rk",
      "type": "function",
      "function": {
        "name": "test_b",
        "arguments": "{}"
      }
    }
  ]
}

The all-messages node in rivet just returns one function_call instead of both:

{
  "type": "chat-message[]",
  "value": [
    {
      "type": "user",
      "message": "Use both functions at the same time"
    },
    {
      "type": "assistant",
      "message": "",
      "function_call": {
        "name": "test_a",
        "arguments": "{}",
        "id": "call_tB0RYGG2UkAS0YKGFFwh1bQz"
      }
    }
  ]
}

So later when we want to send the message back to OpenAI, we cannot reach the proper format necessary. This is a proper example that works fine.

{
    "messages": [
        {
            "role": "user",
            "content": "Use \"Test A\" and \"Test B\" function at the same time"
        },
        {
            "role": "assistant",
            "content": null,
            "tool_calls": [
                {
                    "id": "call_KNWJCI2h3vQD3q7gz63M9aQx",
                    "type": "function",
                    "function": {
                        "name": "test_a",
                        "arguments": "{}"
                    }
                },
                {
                    "id": "call_TrWFBOwx1NrfZt4ESx36PdJj",
                    "type": "function",
                    "function": {
                        "name": "test_b",
                        "arguments": "{}"
                    }
                }
            ]
        },
        {
            "role": "tool",
            "content": "Success",
            "name": "test_a",
            "tool_call_id": "call_KNWJCI2h3vQD3q7gz63M9aQx"
        },
        {
            "role": "tool",
            "content": "Success",
            "name": "test_b",
            "tool_call_id": "call_TrWFBOwx1NrfZt4ESx36PdJj"
        }
    ],
    "model": "gpt-3.5-turbo-1106",
    "temperature": 0.5,
    "max_tokens": 1024,
    "stream": false
}

What was the expected functionality?

Parallel function calling results can be returned properly
all-messages returns all information

Describe your environment

MacOS, Node 14.0.0, etc.

Relevant log output

No response

Relevant screenshots