Fix adding a service when organisation is unknown

An ‘unknown’ organisation can either be:
- one where we know it exists but don’t know much about it (in which
  case the API returns some JSON with the info we do know)
- one we’ve never come across (in which case the API will return `None`)

This commit fixes a bug where we were trying to access the organisation
type in the latter case.

app/models/organisation.py

@@ -25,7 +25,11 @@ class Organisation(JSONModel):
         super().__init__(_dict)
 
         if self._dict == {}:
-            self.name, self.crown, self.agreement_signed, self.domains = None, None, None, []
+            self.name = None
+            self.crown = None
+            self.agreement_signed = None
+            self.domains = []
+            self.organisation_type = None
 
     def as_human_readable(self, fallback_domain):
         if 'dwp.' in ''.join(self.domains):