rpc: add missing space in JSON parsing error message, update test

Summary:
This concludes backport of core#19133 [10/10]
https://github.com/bitcoin/bitcoin/pull/19133/commits/22cb303cf099b430d602384bc92706ce01b4f98d

Depends on D9949

Test Plan: ninja && ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9950