[FIX] models: correct onchange on one2many fields

The method `convert_to_onchange` on one2many fields [1] makes a `copy()`
of `subnames` before altering it. However during onchange management, we
pass it a `PrefixTree` object that can't be copied. As we knows the
object is empty, directly use a empty dict.

[1] https://github.com/odoo/odoo/blob/e6928a6c976cf0f0f7b7b3280205516ad677b413/odoo/fields.py#L2221

Oversight of 2054b59285

odoo/models.py

@@ -5106,7 +5106,7 @@ class BaseModel(MetaModel('DummyModel', (object,), {'_register': False})):
                     continue
                 field = record._fields[name]
                 if not subnames:
-                    result[name] = field.convert_to_onchange(new[name], record, subnames)
+                    result[name] = field.convert_to_onchange(new[name], record, {})
                     continue
                 # x2many fields: serialize value as commands
                 result[name] = commands = [(5,)]