Do not allow to add product or unit with amendment if there was no unit

Invoice lines may already exist with incompatible unit which will fail to
compute invoiced quantity.
(grafted from a261281e016c9445639fd7a47fdf41b8bbb4ab8b)

--HG--
branch : 7.0

modules/purchase_amendment/purchase.py

@@ -250,12 +250,12 @@ class AmendmentLine(ModelSQL, ModelView):
                 & ~(Eval('quantity', 0) < 0),
                 ('purchasable', '=', True),
                 ()),
-            If(Eval('product_uom_category'),
-                ('default_uom_category', '=', Eval('product_uom_category')),
-                ()),
+            ('default_uom_category', '=', Eval('product_uom_category', -1)),
             ],
         states={
-            'readonly': Eval('state') != 'draft',
+            'readonly': (
+                (Eval('state') != 'draft')
+                | ~Eval('product_uom_category', None)),
             'invisible': Eval('action') != 'line',
             })
     product_supplier = fields.Many2One(

modules/sale_amendment/sale.py

@@ -287,12 +287,12 @@ class AmendmentLine(ModelSQL, ModelView):
                 & ~(Eval('quantity', 0) < 0),
                 ('salable', '=', True),
                 ()),
-            If(Eval('product_uom_category'),
-                ('default_uom_category', '=', Eval('product_uom_category')),
-                ()),
+            ('default_uom_category', '=', Eval('product_uom_category', -1)),
             ],
         states={
-            'readonly': Eval('state') != 'draft',
+            'readonly': (
+                (Eval('state') != 'draft')
+                | ~Eval('product_uom_category', None)),
             'invisible': Eval('action') != 'line',
             })
     quantity = fields.Float(