Allow shipping_address being 'None' for an order

I've had an issue with orders that contain only items that don't require
shipping. In this case, the shipping address could be ``None`` which
breaks the ``create_shipping_address`` method.

I've added a test case for it and provided a simple fix. I wasn't quit
sure if the check should be in ``create_shipping_address`` or in
``place_order`` and only call it for an address that is not ``None``.
I've decided to go with the first because it is also simpler to test
this in isolation but am not fussed about moving it up.

oscar/apps/checkout/mixins.py

@@ -145,6 +145,10 @@ def create_shipping_address(self, user, shipping_address):
         Compared to self.get_shipping_address(), ShippingAddress is saved and
         makes sure that appropriate UserAddress exists.
         """
+        # For an order that only contains items that don't require shipping we
+        # won't have a shipping address, so we have to check for it.
+        if not shipping_address:
+            return None
         shipping_address.save()
         if user.is_authenticated():
             self.update_address_book(user, shipping_address)

tests/unit/checkout/mixins_tests.py

@@ -0,0 +1,13 @@
+import mock
+
+from django.test import TestCase
+
+from oscar.apps.checkout.mixins import OrderPlacementMixin
+
+
+class TestOrderPlacementMixin(TestCase):
+
+    def test_can_create_shipping_address_for_empty_address(self):
+        address = OrderPlacementMixin().create_shipping_address(
+            user=mock.Mock(), shipping_address=None)
+        self.assertEquals(address, None)