Prevent errors in case of public methods without validator

[lmignon]

If a public method is declared into a rest.service without validator, an error will be raised when accessing the swagger definition and prevent it's normal display.

We should at least avoid this kind of crash and display a proper error.
Thinking further, I think we should put in place a mechanism that detects this type of inconsistency (public method without a validator) and prevents Odoo from starting in such cases.

[github-actions]

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