feat: ignore 'voice'-parameter when empty (omnivoice)

[Arjenlodder]

I'm using Audio.cpp with Llama-swap. I have an OmniVoice configuration, with voice reference. This works great, but only when the 'voice'-parameter in the json-request is not present. When it's present (even if its empty or NULL), the reference voice is not used and I get random voices. It would be great if this can be fixed, I expect the voice-parameter to do nothing at all, when a reference voice is configured. At the least I would expect that I can use an empty string or a NULL value, to use the reference voice.

[0xShug0]

@Arjenlodder Fixed in #1086