Fix uninitialized URI in batch RPC requests

Summary:
This fixes "Wallet file not specified" errors when making batch wallet RPC
calls with more than one wallet loaded. This issue was reported by
NicolasDorier <nicolas.dorier@gmail.com>
https://github.com/bitcoin/bitcoin/issues/11257

Request URI is not used for anything except multiwallet request dispatching, so
this change has no other effects.

Fixes #11257

Backport core's PR11277

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1185