Fix jinja error on case-insensitive roles and 0-len messages result

[Reithan]

Code is not detecting "Assistant" from ST.

When it is fixed, Jinja template errors consistently if there is only 1 assistant message, due to Jinja formatting failed: list object has no element 0

Both fixed

[LostRuins]

Why >1 and not >=1? Prefilling a single turn should be legal too.

[Reithan]

If == 1, after you run the prefill, the turn array is empty and Jinja throws an error.

Jinja formatting failed: list object has no element 0

[Reithan]

Possibly there's a better resolution for this, because the variability on what the jinja template you're using may or may not support is pretty big.