CLI: Don't check the model head when there is no model head
# What does this PR do?

As the title says -- when there is no model head, don't check it :)

When `architectures` is not defined, it is implied that there is no model head in the weights.
cc @ChrisFugl (related to #18678 )<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_18733). All of your documentation changes will be reflected on that endpoint.<|||||>OK, so it was empty dict, and we can't take max of the values, right? LGTM, thank you @gante <|||||>nit: I would probably check if the dict is empty or not, instead of check `architectures` is None.

For example, `BertModel` has `pooler_output`.<|||||>@ydshieh that is a great point! 

Going to change it<|||||>@ydshieh added your suggestion (plus a check -- a model specified head should have outputs beyond `hidden_...`)