[gen] Minor updates.

fields/__init__.py

@@ -687,7 +687,7 @@ class Field:
                     # because it can be raised as the result of reindexing
                     # the object in situations that are not foreseen by
                     # method in self.default.
-                    return None
+                    return
             else:
                 return self.default
         return value

gen/mixins/ToolMixin.py

@@ -1098,7 +1098,7 @@ class ToolMixin(BaseMixin):
         # Try to authenticate this user
         user = self.authenticate(login, password, request)
         emergency = self._emergency_user
-        if emergency and user is emergency:
+        if emergency and (user is emergency):
             # It is the emergency user.
             return emergency.__of__(self)
         elif user is None: