Allow updating locationless postcodes

- We needed to import a postcode apparently without a location, and
  the import was failing on this one specific postcode (HD7 9AS)
  because `pc.location` was blank. We've made this check more robust,
  and in the case where a postcode is blank, we've saved it anyway
  with the `location` data that comes from the file.

mapit/management/commands/mapit_import_postal_codes.py

@@ -130,23 +130,28 @@ def do_postcode(self, location=None, srid=None):
         try:
             pc = Postcode.objects.get(postcode=self.code)
             if location:
-                curr_location = (pc.location[0], pc.location[1])
-                if settings.MAPIT_COUNTRY == 'GB':
-                    if pc.postcode[0:2] == 'BT':
-                        curr_location = pc.as_irish_grid()
-                    else:
-                        pc.location.transform(27700)  # Postcode locations are stored as WGS84
-                        curr_location = (pc.location[0], pc.location[1])
-                    curr_location = tuple(map(round, curr_location))
-                elif srid != 4326:
-                    pc.location.transform(srid)  # Postcode locations are stored as WGS84
+                if pc.location:
                     curr_location = (pc.location[0], pc.location[1])
-                if curr_location[0] != location[0] or curr_location[1] != location[1]:
+                    if settings.MAPIT_COUNTRY == 'GB':
+                        if pc.postcode[0:2] == 'BT':
+                            curr_location = pc.as_irish_grid()
+                        else:
+                            pc.location.transform(27700)  # Postcode locations are stored as WGS84
+                            curr_location = (pc.location[0], pc.location[1])
+                        curr_location = tuple(map(round, curr_location))
+                    elif srid != 4326:
+                        pc.location.transform(srid)  # Postcode locations are stored as WGS84
+                        curr_location = (pc.location[0], pc.location[1])
+                    if curr_location[0] != location[0] or curr_location[1] != location[1]:
+                        pc.location = location
+                        pc.save()
+                        self.count['updated'] += 1
+                    else:
+                        self.count['unchanged'] += 1
+                else:
                     pc.location = location
                     pc.save()
                     self.count['updated'] += 1
-                else:
-                    self.count['unchanged'] += 1
             else:
                 self.count['unchanged'] += 1
         except Postcode.DoesNotExist: