Fix unlocked_until in getwalletinfo rpc

Summary: Getwalletinfo returned an uninitialized value for unlocked_until before the first walletpassphrase call, even though the wallet was locked and it should return 0.

Test Plan:
make check
./wallet_encryption.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2633