[soc2009/model-validation] Do not run simple validators on empty values

git-svn-id: http://code.djangoproject.com/svn/django/branches/soc2009/model-validation@11033 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/forms/fields.py

@@ -121,6 +121,8 @@ class Field(object):
             raise ValidationError(self.error_messages['required'])
 
     def run_validators(self, value):
+        if value in validators.EMPTY_VALUES:
+            return
         errors = []
         for v in self.validators:
             # don't run complex validators since they need all_values