Fixed #14951 -- Made the unique_for_{date,month,year} model field constraints to not fail when the related DateField is empty.

Existing modelforms tests were extended to cover this case and an equivalent set of tests was added for the model functionality.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15167 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/base.py

@@ -648,7 +648,7 @@ class Model(object):
         called from a ModelForm, some fields may have been excluded; we can't
         perform a unique check on a model that is missing fields involved
         in that check.
-        Fields that did not validate should also be exluded, but they need
+        Fields that did not validate should also be excluded, but they need
         to be passed in via the exclude argument.
         """
         if exclude is None:
@@ -740,6 +740,8 @@ class Model(object):
             # there's a ticket to add a date lookup, we can remove this special
             # case if that makes it's way in
             date = getattr(self, unique_for)
+            if date is None:
+                continue
             if lookup_type == 'date':
                 lookup_kwargs['%s__day' % unique_for] = date.day
                 lookup_kwargs['%s__month' % unique_for] = date.month