Support empty system prompts to use customized system prompts from the server.

[rankaiyx]

I am running multiple local llms through llama.cpp, vllm, and mlc.

When I switch between different models on the server, the server program imports customized system prompts from the model file.

Current client system prompts override server system prompts, resulting in failure to achieve the best experience.

[Taewan-P]

Hello, this feature is now implemented in v0.7.0. Thanks!