Remove required on before/after carrier column of incoterm

We must rely only on the selection validation to require a value to permit to
create new incoterm in incoterm module when carrier_carriage is already
activated.

Closes #14440

doc/migration.rst

@@ -17,6 +17,18 @@ a database from another series.
 7.8
 ---
 
+Before
+~~~~~~
+
+* If ``carrier_carriage`` module is activated, remove ``NOT NULL`` to
+  ``before_carrier`` and ``after_carrier``:
+
+  .. code-block:: SQL
+
+     ALTER TABLE "incoterm_incoterm" ALTER COLUMN "before_carrier" DROP NOT NULL;
+     ALTER TABLE "incoterm_incoterm" ALTER COLUMN "after_carrier" DROP NOT NULL;
+
+
 After
 ~~~~~
 

modules/carrier_carriage/incoterm.py

@@ -11,10 +11,19 @@ class Incoterm(metaclass=PoolMeta):
     before_carrier = fields.Selection([
             ('buyer', "Buyer"),
             ('seller', "Seller"),
-            ], "Before Carrier", required=True,
+            ], "Before Carrier",
         help="Who contracts carriages before main carriage.")
     after_carrier = fields.Selection([
             ('buyer', "Buyer"),
             ('seller', "Seller"),
-            ], "After Carrier", required=True,
+            ], "After Carrier",
         help="Who contracts carriages after main carriage.")
+
+    @classmethod
+    def __register__(cls, module_name):
+        table_h = cls.__table_handler__(module_name)
+        super().__register__(module_name)
+        # Migration from 7.6: remove required on before/after_carrier
+        # to permit to create new incoterm in incoterm module
+        table_h.not_null_action('before_carrier', 'remove')
+        table_h.not_null_action('after_carrier', 'remove')