Optimize fetch_thumbnails (#3624)

In case of a redis backend, this should prevent doing a redis pipeline that looks for a thumbnail
for image "" many times, for each empty image field (such as a member without profile picture).

Consider for example this (partial) redis pipeline call from sentry:

```
"HGETALL 'djthumbs:thumbnails:avatars/98a3dd0519bae43a2bfc45f2bb8653a0.jpg'",
"HGETALL 'djthumbs:thumbnails:'",
"HGETALL 'djthumbs:thumbnails:'",
"HGETALL 'djthumbs:thumbnails:avatars/1152ba8439c354c207967279fcbf4fcb.jpg'",
"HGETALL 'djthumbs:thumbnails:'",
// ... (45 more)
```

website/utils/media/services.py

@@ -89,6 +89,9 @@ def fetch_thumbnails(images: list, sizes=None):
     :param sizes: A list of sizes to prefetch. If None, all sizes will be prefetched.
     :return: None
     """
+    # Filter out empty ImageFieldFiles.
+    images = list(filter(bool, images))
+
     if not images:
         return