Correct an issue with saving tags when no tags were includded in a form.

taggit/tests/tests.py

@@ -88,6 +88,11 @@ class TaggableFormTestCase(BaseTaggingTest):
         apple = Food.objects.get(name='apple')
         self.assert_tags_equal(apple.tags.all(), ['green', 'red', 'yummy', 'delicious'])
         self.assertEqual(Food.objects.count(), 1)
+        
+        f = FoodForm({"name": "raspberry"})
+        raspberry = f.save()
+        self.assert_tags_equal(raspberry.tags.all(), [])
+
 
 class SimilarityByTagTestCase(BaseTaggingTest):
     def test_similarity_by_tag(self):
@@ -105,6 +110,7 @@ class SimilarityByTagTestCase(BaseTaggingTest):
         self.assertEqual(similar_objs, [pear, watermelon])
         self.assertEqual(map(lambda x: x.similar_tags, similar_objs), [3, 2])
 
+
 class TagReuseTestCase(BaseTaggingTest):
     def test_tag_reuse(self):
         apple = Food.objects.create(name="apple")

taggit/utils.py

 from functools import wraps
 
 def parse_tags(tags):
+    if tags is None:
+        tags = ""
     return [o.strip() for o in tags.split(',') if o.strip()]
 
 def require_instance_manager(func):