HAL JSON uses JSON, which uses UTF-8, but properties of ERP5 documents does not enforce any specific encoding, so if we happen to have documents with string properties that are not UTF-8 compatible, they could not be queried with HAL JSON (json.dumps was raising UnicodeError)
This pre-process field default properties and document title to make sure they can be serialized as json. This assumes that configuration (form definition, actions, ...) will be valid UTF-8, but just does extra effort to make sure "broken" data can be displayed without error.