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`
