llama: fix error on bad grammar

[JohannesGaessler]

On master passing a bad grammar to either llama-cli or the server currently does not result in an error, there is only a print indicating that the grammar could not be parsed and the grammar is otherwise ignored. However, I don't think that this is the intended behavior. The code checks if the return value of common_sampler_init is null, it's just that the return value is not null when passing a bad grammar. This PR makes it so that llama_sampler_init_grammar_impl returns null when grammar-str is not empty but could not be passed, this is then propagated upwards to return null with common_sampler_init. When passing a bad grammar to the server the server now returns an error, llama-cli terminates with an error message.

I used the following commands for testing:

export model_name=llama_3-8b && export quantization=q4_0
./build/bin/llama-cli --model models/opt/${model_name}-${quantization}.gguf -ngl 99 --n-predict 1 --prompt "1 2 3 4 " --grammar "root ::= \"6\""
./build/bin/llama-cli --model models/opt/${model_name}-${quantization}.gguf -ngl 99 --n-predict 1 --prompt "1 2 3 4 " --grammar "asdf"
./build/bin/llama-server --model models/opt/${model_name}-${quantization}.gguf --n-gpu-layers 99
curl --request POST --url http://localhost:8080/completion --data '{"prompt": "1 2 3 4 ", "n_predict": 1, "grammar": "root ::= \"6\""}' | python3 -m json.tool
curl --request POST --url http://localhost:8080/completion --data '{"prompt": "1 2 3 4 ", "n_predict": 1, "grammar": "asdf"}' | python3 -m json.tool