Avoid unneeded work if auto-join rooms aren't configured. (#15262)

It is not necessary to reach out to the database to check some
parameters if the auto-join rooms are not configured, or (in some cases)
if auto-create rooms is not configured.

changelog.d/15262.misc

@@ -0,0 +1 @@
+Skip processing of auto-join room behaviour if there are not auto-join rooms configured.

synapse/handlers/register.py

@@ -596,14 +596,20 @@ class RegistrationHandler:
         Args:
             user_id: The user to join
         """
+        # If there are no rooms to auto-join, just bail.
+        if not self.hs.config.registration.auto_join_rooms:
+            return
+
         # auto-join the user to any rooms we're supposed to dump them into
 
         # try to create the room if we're the first real user on the server. Note
         # that an auto-generated support or bot user is not a real user and will never be
         # the user to create the room
         should_auto_create_rooms = False
-        is_real_user = await self.store.is_real_user(user_id)
-        if self.hs.config.registration.autocreate_auto_join_rooms and is_real_user:
+        if (
+            self.hs.config.registration.autocreate_auto_join_rooms
+            and await self.store.is_real_user(user_id)
+        ):
             count = await self.store.count_real_users()
             should_auto_create_rooms = count == 1