fix(data): add input validation for empty prompt/response in make_seq2seq_fields

[Ashutosh0x]

Summary

This PR adds input validation to the make_seq2seq_fields function to handle empty prompt/response arrays gracefully.

Problem

When an empty prompt or response array is passed to make_seq2seq_fields, the function crashes with confusing numpy errors (negative array dimensions) instead of providing a clear error message.

Fixes #483

Solution

Added validation at the start of the function that:

• Checks if prompt is empty and raises ValueError with clear message
• Checks if
  esponse is empty and raises ValueError with clear message

Changes

File: gemma/gm/data/_functional.py

• Added input validation for empty arrays
• Updated docstring to document the new Raises section

Testing

• Syntax validation passed
• The fix provides clear error messages instead of cryptic numpy errors