Fixes #4852

Bug:

After create an empty ledger by sub/unsub and trim ledgers, the first position in managedLedger is ahead of the last when create a consumer without reopen the managed ledger.

Reason：

Last position will be update when opening managedLedger.
Because when creating an empty Ledger without reopening managedLedger, the last position is not be updated. Since the ledger pointed of the last position has been deleted, the first position point to the new ledger.

Fix:

The first position should fall back to last position.