ManyToMany relation with null==True fails in Python 3

After a long term debugging I think, you can reproduce this with my test. I used Python 3.3.5.
```bash
(env)norman@mymachine:~/django-rest-framework$ python --version
Python 3.3.5
(env)norman@mymachine:~/django-rest-framework$ pip freeze
Django==1.6.6
cov-core==1.14.0
coverage==3.7.1
flake8==2.2.2
mccabe==0.2.1
pep8==1.5.7
py==1.4.23
pyflakes==0.8.1
pytest==2.5.2
pytest-cov==1.6
pytest-django==2.6
(env)norman@mymachine:~/django-rest-framework$ ./runtests.py
==== 1 failed, 582 passed, 64 skipped in 4.94 seconds ====
```
The problem only appears, when you use null=True and blank=True on your ManyToManyField. Any idea?