Fix #1351

* Fix TypeError when getting the tags property from an image that has
  no tags. Ex: An image pulled by cryptohash. It is handled like when
  the image doesn't have defined the RepoTags member.

Signed-off-by: Alejandro E. Brito Monedero <alejandro.monedero@gmail.com>

docker/models/images.py

@@ -30,10 +30,10 @@ class Image(Model):
         """
         The image's tags.
         """
-        return [
-            tag for tag in self.attrs.get('RepoTags', [])
-            if tag != '<none>:<none>'
-        ]
+        tags = self.attrs.get('RepoTags')
+        if tags is None:
+            tags = []
+        return [tag for tag in tags if tag != '<none>:<none>']
 
     def history(self):
         """

tests/unit/models_images_test.py

@@ -83,6 +83,11 @@ class ImageTest(unittest.TestCase):
         })
         assert image.tags == []
 
+        image = Image(attrs={
+            'RepoTags': None
+        })
+        assert image.tags == []
+
     def test_history(self):
         client = make_fake_client()
         image = client.images.get(FAKE_IMAGE_ID)