Add fields into vary even if they dont have a value

wagtail/wagtailimages/models.py

@@ -303,9 +303,8 @@ class Filter(models.Model):
 
         for operation in self.operations:
             for field in getattr(operation, 'vary_fields', []):
-                value = getattr(image, field)
-                if value is not None:
-                    vary.append(str(value))
+                value = getattr(image, field, '')
+                vary.append(str(value))
 
         return vary
 

wagtail/wagtailimages/tests/test_image_operations.py

@@ -333,7 +333,7 @@ class TestVaryKey(unittest.TestCase):
         fil = Filter(spec='fill-100x100')
         vary_key = fil.get_vary_key(image)
 
-        self.assertEqual(vary_key, '')
+        self.assertEqual(vary_key, '2e16d0ba')
 
     def test_vary_key_fill_filter_with_focal_point(self):
         image = Image(