fix: Fix error message if there is empty string in Gemini message

[roman-romanov-o]

Currently if user will send messages like this (which is possible in chat if we stop model generation right after the first message, and run one more query after it)

{
  "messages": [
    {
      "content": "hi, what's your name?",
      "role": "user"
    },
    {
      "content": "",
      "role": "assistant"
    },
    {
      "content": "please come again?",
      "role": "user"
    }
  ]
}

Response is

{"error":{"message":"{\"error\":{\"message\":\"Invalid argument: 400 Unable to submit request because it must include at least one parts field, which describes the prompt input. Learn more: [https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini\",\"type\":\"invalid_request_error\",\"code\":\"400\"}}","type":"runtime_error","code":"400"}}](https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini/%22,/%22type/%22:/%22invalid_request_error/%22,/%22code/%22:/%22400/%22%7D%7D%22,%22type%22:%22runtime_error%22,%22code%22:%22400%22%7D%7D)

---

Changed it to same ValidationError("System message content must be present") / ValidationError("User message content must be present") which we raise if content is None

[roman-romanov-o]

/deploy-review

Environment URL: https://chat-ai-dial-adapter-vertexai-pr-164.nightly-test.deltixhub.io
E2E tests status: success