FIX : handle empty choices in Doc.get_golds() to prevent IndexError

[nandeanie]

Fixes #1162

Problem

When a generative task sets choices=[ ] with gold_index=0,
get_golds() raised an unhelpful IndexError: list index out of range.

Fix

Added a check in get_golds( ) to return empty list if choices is empty,
and a clear ValueError if gold_index is out of range.

Testing

Verified fix with:
doc = Doc(query='test', choices=[ ], gold_index=0)
doc.get_golds() # now returns [ ] instead of crashing