Should allow empty prompt in `$chat_structured()`

```R
chat <- chat_anthropic()

chat$chat(
  content_image_file("holly-mandarich-3p9zaNwUtv8-unsplash.jpg"),
  "Describe this photo"
)

chat$chat_structured("", type = type_object(
  person = type_boolean("Is there a person in the photo?"),
  landscape = type_string("Briefly describe the landscape")
))
```