fix(predict): handle temperature=0 correctly instead of treating it as unset

[isaacbmiller]

temperature=0 is a valid and common setting for deterministic generation,
but the or operator treats 0 as falsy. This causes the user's explicit
temperature=0 to be silently dropped and replaced with the LM's default.

When num_generations > 1, this leads to the temperature being overridden
to 0.7, giving random outputs when the user explicitly requested
deterministic generation.

Fix: use explicit None check instead of or.