Make vary_key a blank string if there are no vary fields

This addresses an issue which causes all renditions in a project to be regenerated after an upgrade to Wagtail 0.9.

wagtail/wagtailimages/models.py

@@ -311,10 +311,12 @@ class Filter(models.Model):
 
     def get_vary_key(self, image):
         vary_string = '-'.join(self.get_vary(image))
-        vary_key = hashlib.sha1(vary_string.encode('utf-8')).hexdigest()
 
-        return vary_key[:8]
+        # Return blank string if there are no vary fields
+        if not vary_string:
+            return ''
 
+        return  hashlib.sha1(vary_string.encode('utf-8')).hexdigest()[:8]
 
     _registered_operations = None
 

wagtail/wagtailimages/tests/test_image_operations.py

@@ -326,14 +326,14 @@ class TestVaryKey(unittest.TestCase):
         fil = Filter(spec='max-100x100')
         vary_key = fil.get_vary_key(image)
 
-        self.assertEqual(vary_key, 'da39a3ee')
+        self.assertEqual(vary_key, '')
 
     def test_vary_key_fill_filter(self):
         image = Image(width=1000, height=1000)
         fil = Filter(spec='fill-100x100')
         vary_key = fil.get_vary_key(image)
 
-        self.assertEqual(vary_key, 'da39a3ee')
+        self.assertEqual(vary_key, '')
 
     def test_vary_key_fill_filter_with_focal_point(self):
         image = Image(