[OpenSIPS-Devel] [OpenSIPS/opensips] 1a8f8e: Fix strange `"error":null` JSON node

[Bogdan Andrei IANCU]

  Branch: refs/heads/master
  Home:   https://github.com/OpenSIPS/opensips
  Commit: 1a8f8e47bf3badc68927fbb573b1e78c596d2055
      https://github.com/OpenSIPS/opensips/commit/1a8f8e47bf3badc68927fbb573b1e78c596d2055
  Author: Bogdan-Andrei Iancu <bogdan at opensips.org>
  Date:   2023-04-10 (Mon, 10 Apr 2023)

  Changed paths:
    M modules/jsonrpc/jsonrpc.c

  Log Message:
  -----------
  Fix strange `"error":null` JSON node

An `error` node with NULL value does not make much of a sense, but let's try to handle it.
Closes #3047