JsonSchemaValidator raises IndexError for an empty message list

Describe the bug

JsonSchemaValidator.run([]) raises a bare IndexError while trying to read messages[-1]. The component accepts a list of messages and documents validation errors, but it does not validate that the list contains a message before indexing it.

Error message

IndexError: list index out of range

Expected behavior

An empty message list should raise a descriptive ValueError, consistent with the component's existing validation errors for a missing schema or a message with no text content.

To Reproduce

from haystack.components.validators import JsonSchemaValidator

validator = JsonSchemaValidator(json_schema={"type": "object"})
validator.run([])

Additional context

The failure happens before schema validation at last_message = messages[-1]. A guard at the start of run() can turn the implementation detail into an actionable input error.

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System:

• OS: Linux
• Haystack version: main at 44246f850714529353513a2a41e691bb1cf78b7e