fix dumpprivkeys, wallet.get_private_key and export_privkeys_dialog

Don't show private key for the next_account.
Wallet does not return the private key for PendingAccount, but an empty list instead.
It could be implemented, but I don't see the need.

See https://github.com/spesmilo/electrum/issues/884 and https://github.com/spesmilo/electrum/issues/935.
Overrides https://github.com/spesmilo/electrum/pull/936