Fixed #17294 -- Supported nullable DateTimeFields when time zone support is enabled. Thanks pressureman for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17148 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/utils/timezone.py

@@ -228,6 +228,9 @@ def now():
     else:
         return datetime.now()
 
+# By design, these four functions don't perform any checks on their arguments.
+# The caller should ensure that they don't receive an invalid value like None.
+
 def is_aware(value):
     """
     Determines if a given datetime.datetime is aware.