[FIX] base: display correct ondelete value

The on_delete value on an ir.model.field did not reflect the value set on the
field loaded in the registry

Fixes odoo/odoo#24464

closes odoo/odoo#27906

odoo/addons/base/ir/ir_model.py

@@ -746,6 +746,7 @@ class IrModelFields(models.Model):
             'index': bool(field.index),
             'store': bool(field.store),
             'copy': bool(field.copy),
+            'on_delete': getattr(field, 'ondelete', None),
             'related': ".".join(field.related) if field.related else None,
             'readonly': bool(field.readonly),
             'required': bool(field.required),