Fix TF Flaubert and XLM
# What does this PR do?

By doing some experiments on Flaubert and XLM I realized that building a model with a `None` argument forces this value to be `None`  when served. Then to fix this issue, the build takes a proper input without a `None` value.

I'm checking what is going wrong as the tests of equivalence should not fail.<|||||>Ok, now it works 👍 