createsuperuser / get_default_username fail on systems without default locale encoding

[Preston Holmes]

r16182 introduced a regression in the case of systems that, for whatever reason, don't provide a locale encoding.

For example on OS X Server 10.6:

>>> import locale
>>> locale.getdefaultlocale()
(None, None)

as documented for the locale module, None is a valid return value:

​http://docs.python.org/library/locale.html#locale.getdefaultlocale

However, the decode function does not allow None and will raise a TypeError:

>>> get_default_username()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/private/tmp/foo/src/django/django/contrib/auth/management/__init__.py", line 105, in get_default_username
    default_username = get_system_username()
  File "/private/tmp/foo/src/django/django/contrib/auth/management/__init__.py", line 85, in get_system_username
    return getpass.getuser().decode(locale.getdefaultlocale()[1])
TypeError: decode() argument 1 must be string, not None

the attach patch should fix this for most cases, since UTF8 is the best default

however, short of mocking locale.getdefaultlocale, I'm not sure how to test this.

[Karen Tracey]

Already reported in #16017