models: Fetch "recipient" object when along with "Huddle" object.

We now fetch recipient object when querying "Huddle" object in
get_or_create_huddle_backend as this query is eventually used
to get the recipient object only in get_huddle_recipient.

This commit also updates the select_related call in the code to
populate Huddle objects in cache to pass "Recipient" as argument.
Previously no argument was passed to select_related and thus no
related objects were being fetched, with no non-null related fields
being present.

zerver/lib/cache_helpers.py

@@ -98,7 +98,7 @@ cache_fillers: Dict[
         10000,
     ),
     "huddle": (
-        lambda: Huddle.objects.select_related().all(),
+        lambda: Huddle.objects.select_related("recipient").all(),
         huddle_cache_items,
         3600 * 24 * 7,
         10000,

zerver/models.py

@@ -4074,7 +4074,9 @@ def get_or_create_huddle(id_list: List[int]) -> Huddle:
 )
 def get_or_create_huddle_backend(huddle_hash: str, id_list: List[int]) -> Huddle:
     with transaction.atomic():
-        (huddle, created) = Huddle.objects.get_or_create(huddle_hash=huddle_hash)
+        (huddle, created) = Huddle.objects.select_related("recipient").get_or_create(
+            huddle_hash=huddle_hash
+        )
         if created:
             recipient = Recipient.objects.create(type_id=huddle.id, type=Recipient.HUDDLE)
             huddle.recipient = recipient