Fixed #18829 -- Fixed ModelChoiceIterator length

Thanks facundo.olano at gmail.com for the report and thikonom for
the initial patch.

django/forms/models.py

@@ -917,7 +917,8 @@ class ModelChoiceIterator(object):
                 yield self.choice(obj)
 
     def __len__(self):
-        return len(self.queryset)
+        return len(self.queryset) +\
+                    (1 if self.field.empty_label is not None else 0)
 
     def choice(self, obj):
         return (self.field.prepare_value(obj), self.field.label_from_instance(obj))

tests/modeltests/model_forms/tests.py

@@ -989,6 +989,7 @@ class OldFormForXTests(TestCase):
             (c2.pk, "It's a test"),
             (c3.pk, 'Third'),
             (c4.pk, 'Fourth')])
+        self.assertEqual(5, len(f.choices))
         with self.assertRaises(ValidationError):
             f.clean('')
         with self.assertRaises(ValidationError):