bugdown: Avoid ORM when there are no group names.

This change avoids hitting the Django ORM when
we don't find any possible group mentions in
the message content.

Django doesn't necessarily actually hit the database,
but it's still slow and shows up in profiles.

zerver/lib/bugdown/__init__.py

@@ -1846,12 +1846,22 @@ class MentionData:
             row['id']: row
             for row in self.full_name_info.values()
         }
+        self.init_user_group_data(realm_id=realm_id, content=content)
 
+    def init_user_group_data(self,
+                             realm_id: int,
+                             content: str) -> None:
         user_group_names = possible_user_group_mentions(content)
         self.user_group_name_info = get_user_group_name_info(realm_id, user_group_names)
-        group_ids = [group.id for group in self.user_group_name_info.values()]
-        membership = UserGroupMembership.objects.filter(user_group_id__in=group_ids)
         self.user_group_members = defaultdict(list)  # type: Dict[int, List[int]]
+        group_ids = [group.id for group in self.user_group_name_info.values()]
+
+        if not group_ids:
+            # Early-return to avoid the cost of hitting the ORM,
+            # which shows up in profiles.
+            return
+
+        membership = UserGroupMembership.objects.filter(user_group_id__in=group_ids)
         for info in membership.values('user_group_id', 'user_profile_id'):
             group_id = info['user_group_id']
             user_profile_id = info['user_profile_id']