mixtral:8x22b model does not work with system prompt only

### What is the issue? The `mixtral:8x22b-instruct` model does not work correctly when only the system prompt is provided. In such cases, an empty prompt is sent, leading to irrelevant output. This behavior may be related to the internal handling of prompts or recent changes made in the system prompt handling, as referenced in #4228. mixtral:8x22b-instruct template: https://ollama.com/library/mixtral:8x22b-instruct/blobs/138b3322e0da Mixtral 8x22B template in docs: https://github.com/ollama/ollama/blob/main/docs/template.md#mistral #### Steps to Reproduce 1. Input the following system prompt only into the `mixtral:8x22b-instruct` model. ```bash curl http://localhost:11434/api/chat -d '{ "model": "mixtral:8x22b-instruct-v0.1-q4_K_M", "stream": false, "messages": [ { "role": "system", "content": "Hello, I am Ollama. I am here to help you with your questions. What would you like to know?\n\nWhat is the capital of Japan?" } ] }' ``` 2. The Ollama log shows that the `prompt` field is empty. `prompt=\"\"\r\n` ```bash {"log":"time=2024-10-04T08:03:17.462Z level=DEBUG source=routes.go:1417 msg=\"chat request\" images=0 prompt=\"\"\r\n","stream":"stdout","time":"2024-10-04T08:03:17.462471779Z"} ``` 4. The output is unrelated to the input content. ```bash {"model":"mixtral:8x22b-instruct-v0.1-q4_K_M","created_at":"2024-10-04T08:03:40.224484198Z","message":{"role":"assistant","content":",\n.\nA new study by the University of Maryland and Johns Hopkins Medicine has found ..."} ``` #### Expected Behavior The `mixtral:8x22b-instruct` model should work with system prompt only, similar to other models like `gemma2:27b`. #### Actual Results mixtral:8x22b-instruct, system prompt only: NG <details><summary>Results</summary> <p> input: ```bash curl http://localhost:11434/api/chat -d '{ "model": "mixtral:8x22b-instruct-v0.1-q4_K_M", "stream": false…