Fix and add test for empty chain and reorg consistency for gettxoutsetinfo.

Summary: backport PR10445 from core. Fix a possible crash.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py blockchain.py

Reviewers: freetrader, CCulianu, #bitcoin_abc

Reviewed By: CCulianu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D502