PYSON not eval correctly empty Time and TimeDelta
A state with Bool(Eval(TimeDelta)) return false in SAO (default is 00:00).
Example:
Project Invoice module: the project_invoice_method field in project.work, the readonly state return false because invoice_duration is TimeDelta field [1]
[1] https://bitbucket.org/tryton/project_invoice/src/c905d1f07729a436b50d0c1784d51b54d6705169/work.py?at=default&fileviewer=file-view-default#work.py-36