[`T5/LlamaTokenizer`] default legacy to `None` to not always warn
# What does this PR do?
In a follow up to the patch that introduce the `legacy` argument #24622, this makes sure people are warn if the `legacy` is not set. Since online models were not changed, this does not really change much! 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Excellent - thank you for improving this feature, Arthur!