Exception retrieving untagged images on api >= 1.24
Docker API >= 1.24 will return a null object if image tags DNE instead of not including it in the response. This makes the dict.get fail to catch the null case and the list comprehension to iterate over a non-iterable.

```
  File "<stdin>", line 1, in <module>
  File "docker/models/images.py", line 16, in __repr__
    return "<%s: '%s'>" % (self.__class__.__name__, "', '".join(self.tags))
  File "docker/models/images.py", line 34, in tags
    tag for tag in self.attrs.get('RepoTags', [])
TypeError: 'NoneType' object is not iterable
```

This is similar to an issue seen in [salt](https://github.com/saltstack/salt/pull/35447/commits/b833b5f9587534d3b843a026ef91abc4ec929d0f)

Was able to get things working with a pretty quick change:
```
diff --git a/docker/models/images.py b/docker/models/images.py
index 32068e6..39a640d 100644
--- a/docker/models/images.py
+++ b/docker/models/images.py
@@ -30,9 +30,11 @@ class Image(Model):
         """
         The image's tags.
         """
+        tags = self.attrs.get('RepoTags', [])
+        if tags is None:
+            return []
         return [
-            tag for tag in self.attrs.get('RepoTags', [])
-            if tag != '<none>:<none>'
+            tag for tag in tags if tag != '<none>:<none>'
         ]
 
     def history(self):
```
diff --git a/docker/models/images.py b/docker/models/images.py
index 32068e69..6f8f4fe2 100644
--- a/docker/models/images.py
+++ b/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):
         """
diff --git a/tests/unit/models_images_test.py b/tests/unit/models_images_test.py
index 392c58d7..efb21166 100644
--- a/tests/unit/models_images_test.py
+++ b/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)
['tests/unit/models_images_test.py', 'docker/models/images.py']