Make value field mandatory for percentage discount benefit.

src/oscar/apps/offer/abstract_models.py

         if not self.range:
             raise exceptions.ValidationError(
                 _("Percentage benefits require a product range"))
+        if not self.value:
+            raise exceptions.ValidationError(
+                _("Percentage discount benefits require a value"))
         if self.value > 100:
             raise exceptions.ValidationError(
                 _("Percentage discount cannot be greater than 100"))

tests/integration/offer/percentage_benefit_tests.py

 from decimal import Decimal as D
 
+from django.core import exceptions
 from django.test import TestCase
 import mock
 
         self.assertEqual(1 * D('4.00') * D('0.2'), result.discount)
         self.assertEqual(3, self.basket.num_items_with_discount)
         self.assertEqual(0, self.basket.num_items_without_discount)
+
+
+class TestAPercentageDiscountBenefit(TestCase):
+
+    def test_requires_a_benefit_value(self):
+        rng = models.Range.objects.create(
+            name="", includes_all_products=True)
+        benefit = models.Benefit.objects.create(
+            type=models.Benefit.PERCENTAGE, range=rng
+        )
+        with self.assertRaises(exceptions.ValidationError):
+            benefit.clean()