convert : fix conversion for Mistral-Medium-3.5-128B (#24268)

Mistral explicitly sets `moe` and `llama_4_scaling` to `null` in
params.json, breaking `key in dict` checks during conversion. Replace
with `dict.get(key) is not None` where this matters.

Fixes `convert-hf-to-gguf.py --mistral-format Mistral-Medium-3.5-128B`

convert_hf_to_gguf.py

@@ -238,7 +238,7 @@ def main() -> None:
             assert hparams.get("vision_encoder") is not None, "This model does not support multimodal"
             from conversion.pixtral import PixtralModel
             model_class = PixtralModel
-        elif "moe" in hparams:
+        elif hparams.get("moe") is not None:
             from conversion.mistral import MistralMoeModel
             model_class = MistralMoeModel
         else: