Fix VertexAI Empty Model Parts Error

[caesarnine]

Fixes this error that occurs with VertexAI Gemini models fairly often in my experience.

Issue that's open with the same error: #2032

ClientError: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': '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', 'status':
'INVALID_ARGUMENT'}}

Due to the last message being something like:

    {
        "parts": [{ "content": "", "part_kind": "text" }],
        "usage": {
            "requests": 1,
            "request_tokens": 25308,
            "response_tokens": 0,
            "total_tokens": 25308,
            "details": {
                "cached_content_tokens": 15171,
                "text_cache_tokens": 15171,
                "text_prompt_tokens": 25308
            }
        },
        "model_name": "gemini-2.5-pro",
        "timestamp": "2025-07-14T17:33:52.568858Z",
        "kind": "response",
        "vendor_details": { "finish_reason": "STOP" },
        "vendor_id": "fz91aPDpJYienvgP757VgQ0"
    }

Tested the change against a session that was previously hard crashing and it works fine now.

[DouweM]

@caesarnine Thanks! Can you please add a test for the scenario that this fixes, to ensure it doesn't accidentally break again in the future?

[caesarnine]

@DouweM Done - let me know if it looks ok (never used pytest-vcr anymore, so feel free to let me know if I did something wrong there).