Once upon a time, a user's Profile.extra_data defaulted to None rather than an

empty dictionary. We recently introduced code that assumes that the field is

non-None, and we will probably continue to add more and more such code in the

future. This change adds some normalization to User._get_profile to make sure

that it's not None.

Ran unit tests. Before the fix, the new test failed. After, it succeeds.