do not cache default callable return value of None in get_or_set

[Dan Tao]

In [82be474 ​https://github.com/django/django/commit/82be474efa81e5f2e127d711045a2ec06b206a8e] the change was made to allow None as a default value passed to
BaseCache.get_or_set. In case the default value of None is used, it is not
stored in the cache. This still left open the possibility that default could
be a callable that returns None, in which case it would be stored in the
cache.

# This scenario works as expected.
cache.get_or_set('foo', None)  # None
cache.get_or_set('foo', 5)  # 5
cache.get('foo')  # 5

# This scenario seems wrong.
cache.get_or_set('bar', lambda: None)  # None
cache.get_or_set('bar', 5)  # None :(
cache.get('bar')  # None :(