Fix max tokens tool calls function

[carlows]

When you use the max_tokens option in google gemini, you get a response that looks like this once the limit is reached:

{
  "candidates": [
    {
      "content": {
        "role": "model"
      },
      "finishReason": "MAX_TOKENS",
      "index": 0
    }
  ],
  "usageMetadata": {
    ...
  },
  "modelVersion": "gemini-2.5-flash-lite",
  "responseId": "123"
}

Then the .tool_calls function fails because nil doesn't have a has_key? method.