A many2one field that is required means that its column cannot contain
null values, however the default ondelete policy for all m2o fields is
`set null`, which contradicts `required=True`.
This patch will, by default, apply the `restrict` policy to required m2o
fields unless the ondelete attribute is explicitly specified. However,
if the policy specified is `set null`, a ValueError will be raised
whenever the registry registers the field as this makes no sense.
The default ondelete policy remains unchanged for non-required m2o
fields (set null)
closesodoo/odoo#30122
Adrian Torres
ee6dc40a60