all instances have user data

Hi,
  I just noticed that all of my instances are showing user-data in the metadata service even if the instance was not started with any user-data. Launch an instance like:

$ img=f57b91f2-3eac-4690-bf2d-cf28b25486e8 ; key="mykey"
$ nova boot sm-foo-user-data --flavor m1.tiny --key_name $key --image $img sm-foo-no-user-data

Then, inside the instance you'll see:
$ wget -O - -q http://169.254.169.254/latest/ ; echo
meta-data/
user-data

Previously (and in EC2) user-data is not listed. This indicates it is Null rather than "".

I see in the database:

mysql> select id, hostname, user_data from instances;
+----+---------------------------------------------+--------------------------+
| id | hostname | user_data |
+----+---------------------------------------------+--------------------------+
| 4 | sm-foo-user-data-none | |
| 6 | sm-foo-user-data | W3B5dGhvbjogKioucHldCgo= |
+----+---------------------------------------------+--------------------------+

I think I would have expected a NULL there for user_data.

I know the metadata service code has explicit checks for this:
nova/api/metadata/base.py:
        if instance.get('user_data', None) is not None:
            self.userdata_b64 = base64.b64decode(instance['user_data'])
        else:
            self.userdata_b64 = None

I'm not actually sure at what point this regressed. It seems present in the folsom nova installation I have access to.
Is there a way to determine that userdata is None rather than "" ?

Perhaps the web service code is just doing it wrong?