Fix false positive

I found out while trying to make another test fail.
If the `new_memberships` array was empty (i.e none of the user memberships were found) it had nothing to iterate over, so it returned true.
Hopefully, only one test seems to be affected by this.

plugins/chat/spec/jobs/regular/auto_join_channel_batch_spec.rb

@@ -201,6 +201,7 @@ describe Jobs::AutoJoinChannelBatch do
 
   def assert_users_follows_channel(channel, users)
     new_memberships = UserChatChannelMembership.where(user: users, chat_channel: channel)
+    expect(new_memberships.length).to eq(users.length)
     expect(new_memberships.all?(&:following)).to eq(true)
   end