Allow LLM router fails with empty preprompt

[canyon289]

Would be great if it could handle cases of empty preprompt

[canyon289]

Preprompt doesnt seem to be needed for some reason

[comath]

The serde derive uses the default of "none" if it's not there in the JSON. This error is due to OpenAI's model's code.

We can make the default the empty string in the trait interface and trait so that we don't have to detect if it's not-none and not-empty. It would make making new models simpler.

[comath]

This is also because you're using preprompt instead of system.

[canyon289]

This is also because you're using preprompt instead of system.

Preprompt is what's used in llm_verification? Should this be changed there?
https://github.com/aivillage/llm_verification/blob/ad0425922616ab7779720f38daf69af5cfcbf8c8/remote_llm.py#L40