Make blank/empty bearer_token *mean* NULL

So it turns out that CharFields & NULL really don't work together, even
if you set null=true (see https://code.djangoproject.com/ticket/9590)

So the first change is a data migration to change any NULL bearer tokens
back to "" after previous commit which allowed NULL.

I've changed the ``serialize()`` method to interpret bearer_token of ""
as NULL. I've added tests and updated the view test so we are sure that
our backdrop endpoint will present NULL, not "".
Merge pull request #49 from alphagov/blank-token-means-null

Make blank/empty bearer token *mean* NULL