Make sure missing values are handled correctly

[adrinjalali]

ref: https://github.com/huggingface/api-inference-community/issues/82

This is probably something which needs to be fixed on the https://github.com/huggingface/api-inference-community or https://github.com/huggingface/hub-docs (widgets) side, but having an issue here to make sure we don't forget about it.