Fixed #13085 -- Don't fail on creation of model with GFK to a model with __len__() returning zero.

Also, according to the comments on the ticket and its duplicates, added
tests execising saving an instance of a model with a GFK to:

* An unsaved object -- This actually doesn't generate the same failure
  but another ORM-level exception. The test verifies it's the case.

* An instance of a model with a __nonzero__() method thant returns False
  for it. This doesn't fail because that code path isn't executed.

* An instance of a model with a CharField PK and an empty value for it.
  This doesn't fail.

1 files changed, 2 insertions, 2 deletions

diff --git a/django/contrib/contenttypes/generic.py b/django/contrib/contenttypes/generic.py
index d849d1607e..20ff7042bc 100644
--- a/django/contrib/contenttypes/generic.py
+++ b/django/contrib/contenttypes/generic.py
@@ -43,7 +43,7 @@ class GenericForeignKey(object):
 
     def instance_pre_init(self, signal, sender, args, kwargs, **_kwargs):
         """
-        Handles initializing an object with the generic FK instaed of
+        Handles initializing an object with the generic FK instead of
         content-type/object-id fields.
         """
         if self.name in kwargs:
@@ -52,7 +52,7 @@ class GenericForeignKey(object):
             kwargs[self.fk_field] = value._get_pk_val()
 
     def get_content_type(self, obj=None, id=None, using=None):
-        if obj:
+        if obj is not None:
             return ContentType.objects.db_manager(obj._state.db).get_for_model(obj)
         elif id:
             return ContentType.objects.db_manager(using).get_for_id(id)