Fixed #1336 -- made USStateField work correctly when data is None (thanks, chrisb)

git-svn-id: http://code.djangoproject.com/svn/django/trunk@2427 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/core/formfields.py

@@ -875,7 +875,10 @@ class USStateField(TextField):
             raise validators.CriticalValidationError, e.messages
 
     def html2python(data):
-        return data.upper() # Should always be stored in upper case
+        if data:
+            return data.upper() # Should always be stored in upper case
+        else:
+            return None
     html2python = staticmethod(html2python)
 
 class CommaSeparatedIntegerField(TextField):