Do not display commission price on stock move if not set

Like the cost fields we should display it only if it is set.

modules/commission/stock.py

@@ -6,6 +6,7 @@ from decimal import Decimal
 from trytond.model import fields
 from trytond.modules.product import price_digits, round_price
 from trytond.pool import Pool, PoolMeta
+from trytond.pyson import Eval
 from trytond.transaction import Transaction
 
 
@@ -13,7 +14,10 @@ class Move(metaclass=PoolMeta):
     __name__ = 'stock.move'
 
     commission_price = fields.Numeric(
-        "Commission Price", digits=price_digits, readonly=True)
+        "Commission Price", digits=price_digits, readonly=True,
+        states={
+            'invisible': ~Eval('commission_price'),
+            })
 
     @classmethod
     def __setup__(cls):