WhisperProcessor::Process doesn't handle payload.prompt being set.

[skottmckay]

Describe the bug

onnxruntime-genai/src/models/whisper_processor.cpp

Line 41 in 053d7d7

std::shared_ptr<Tensor> input_ids = tokenizer.EncodeBatch(payload.prompts);

doesn't check payload.prompt. It should check that payload.prompt or payload.prompts is set and use the correct one.

To Reproduce
Run with a single prompt set.

Expected behavior
No divide by zero error.