[Qt] Use wallet 0 in rpc console if running with multiple wallets

Summary:
Backport of Core PR10870

Completes T441

Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: ninja

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1963